Directions: There is some relationship between the two pair of letters on the left of the sign (::). The same relationship exists between the two terms on the right of which one is missing. Find the missing one from the given alternatives.

OBNP : PDQT : : HPET : ?

This requires analyzing the relationship between OBNP and PDQT to apply to HPET. Looking at the patterns: O(15) -> P(16) is +1, B(2) -> D(4) is +2, N(14) -> Q(17) is +3, P(16) -> T(20) is +4. The pattern increments by 1 each time: +1, +2, +3, +4. Applying this to HPET: H(8)+1=I(9), P(16)+2=R(18), E(5)+3=H(8), T(20)+4=X(24). This gives IRHX, which is option D.