Antonyms

Timorous means showing fear or timidity. Intrepid means fearless and brave - the direct opposite. The other options (agile, obey, opaque) are not related to courage or fear.

Thin describes something lacking bulk or mass. Dense describes something compact, thick, or having high mass per volume - the opposite concept. The other options (bottom, abysmal, albeit) are not related to thinness.

Tear means to rip or damage by pulling apart. Mend means to repair or fix something that is torn or broken - the opposite action. The other options (mould, mermaid, forfeit) are not related to tearing.

Supine means lying face up or horizontal. Erect means upright, vertical, or standing - the opposite position. Note: prone means lying face down, which is also horizontal but not the opposite of supine.

To aggrandize means to make greater, more powerful. Its opposite is to render helpless. 

To aggravate means to worsen, to intensify, to exacerbate. Its opposite is to alleviate.

Aggregate means totality, entirety. Its opposite is isolated units.

To agitate means to shake up, to disquiet. Its opposite is to soothe which means to soften.

Agility means the ability to move quickly and easily. Its opposite is tardiness, which means slowness in motion or action.

Alacrity means prompt and cheerful willingness. Its opposite is hesitance and reluctance.

To alienate means to estrange, to disunite. Its opposite is to reunite.

An ally means a friend, a supporter. Its opposite is an adversary.

To alleviate means to make easier to bear. Its opposite is to exacerbate.

To amortise means to set apart money regularly to pay up a debt. Its opposite among the choices is to add to one's debts.  

Analgesia means the absence of the sense of pain. Its opposite is sensitivity to pain.