Magnetism and Electromagnetism

The north pole of the normal magnet points in the north direction of the earth. Thus by suspending a bar magnet freely, it aligns in the north-south direction.

A freely suspended magnet always points in the north south direction as earth behaves as large magnet. the north pole of the earth's magnet attracts the south pole of the magnet and vice versa.

The compass needle acts as a freely suspended magnet which points in north-south direction under the influence of earth's magnetic field lines. So, a small plotting compass can be used to plot a magnetic field.

When a magnetic compass is brought close to a current carrying wire there is deflection in the compass due to the formation of magnetic field. The magnetic field lines around a long wire which carries an electric current form concentric circles around the wire. The direction of the magnetic field is perpendicular to the wire. 
The deflection increases when the current is increased because the magnitude of the field is directly proportional to the strength of the electric current. That is, stronger the electric field, stronger the magnetic field.

we know,
The time period of vibration magnetometer is,
$T = 2\pi \sqrt {\dfrac{1}{{mH}}} $
For some moment of inertia $I$ and magnetic moment $m$
$T \propto \dfrac{1}{{\sqrt H }}$
T can be reduced if horizontal component of earth's magnetic field $H$ is increased. But
$H = R\cos \theta $
$H$ is high if the dip is zero.
This occurs at the equator.
Hence the period can be reduced by moving towards the equator.

Properties of magnetic lines of force
(i) they always start from north pole and end at the south pole of the magnet outside the magnet.
(ii) they never intersect each other.

In a compass, needle points toward earth's magnetization south thus it its tip is north and its other end is south which is repelled by south pole of other magnet.

A magnetic compass points north because all magnets have two poles, a north pole and a south pole, and the north pole of one magnet is attracted to the south pole of another magnet. 
When this magnetic compass is brought near a magnetic substance, there is change in the direction of the needle based on the polarity of the magnetic substance closer to it. Whereas for a non magnetic substance it is not so.
Hence, the direction in which a compass needle points will distinguish between a magnetic material and a non-magnetic material.

Answer is D.

A freely suspended magnet always points in the North-South direction even in the absence of any other magnet. This suggests that the Earth itself behaves as a magnet which causes a freely suspended magnet (or magnetic needle) to point always in a particular direction: North and South. The shape of the Earth's magnetic field resembles that of a bar magnet of length one-fifth of the Earth's diameter buried at its center.
The South Pole of the Earth's magnet is in the geographical North because it attracts the North Pole of the suspended magnet and vice versa. Thus, there is a magnetic S-pole near the geographical North, and a magnetic N-pole near the geographical South. The positions of the Earth's magnetic poles are not well defined on the globe; they are spread over an area. The axis of Earth's magnet and the geographical axis do no coincide. The axis of the Earth's magnetic field is inclined at an angle of about 15 degrees with the geographical axis. Due to this a freely suspended magnet makes an angle of about 15 degrees with the geographical axis and points only approximately in the North-South directions at a place.

In the Earth, the liquid metal that makes up the outer core passes through a magnetic field which causes an electric current to flow within the liquid metal. The electric current, in turn, creates its own  magnetic field—one that is stronger than the field that created it in the first place.

An iron rod buried inside earth along N-S direction becomes a weak magnet. It is only possible when the earth itself behaves like a magnet.

The field magnet used in a moving coil galvanometer is very strong. The earth's magnetic field is quite weak as compared to the magnetic field produced by the field magnet. Practically the coil rotates under the effect of the strong magnetic field due to the earth does not affect the working of the moving coil galvanometer.