Q.Which of the following species does not exert a resonance effect?

Resonance (mesomerism) occurs in organic compounds with a double bond at a suitable position whose true structure cannot be captured by any single Lewis structure -- such compounds can be drawn as multiple canonical (resonance) structures differing only in the placement of bonding/lone-pair electrons, with the real molecule being a single, fixed resonance hybrid lying between them (not an equilibrium mixture, and not a molecule that flips between forms).

In 1,3-butadiene ($H_2C=CH$-$CH=CH_2$), a single localised structure predicts $C_1$-$C_2$ and $C_3$-$C_4$ should be shorter than the central $C_2$-$C_3$ bond, but all bond lengths are observed nearly equal -- proof that the $\pi$ electrons are delocalised across all four carbons rather than confined to two separate double bonds. Benzene is the other classic case: its true, uniformly-bonded, exceptionally stable structure is only correctly understood as the resonance hybrid of its Kekulé canonical forms. …