Which of the following will not undergo addition reaction?

As per the NCERT chapter, 'Unsaturated hydrocarbons add hydrogen in the presence of catalysts such as palladium or nickel to give saturated hydrocarbons' — this is an addition reaction, and only compounds containing a double or triple bond (unsaturated hydrocarbons) can undergo it.

(A) $C_4H_8$: this formula matches the alkene general formula $C_nH_{2n}$ (butene), which contains a carbon-carbon double bond. Being unsaturated, it can undergo addition reactions (e.g., hydrogenation to butane using a Ni catalyst) — so this is not the answer.

(B) $C_2H_2$: this is ethyne, matching the alkyne general formula $C_nH_{2n-2}$, containing a carbon-carbon triple bond. It readily undergoes addition reactions — so this is not the answer.

(C) $C_3H_8$: this is propane, matching the alkane general formula $C_nH_{2n+2}$. All carbon-carbon bonds in propane are single bonds, so it is a saturated hydrocarbon. As stated in the chapter, saturated compounds 'are normally not very reactive' toward addition and instead undergo substitution reactions (e.g., with chlorine in sunlight). Propane has no double or triple bond to add hydrogen (or other reagents) across, so it will not undergo an addition reaction. This is the correct answer.

(D) $C_2H_4$: this is ethene, matching the alkene general formula $C_nH_{2n}$, containing a carbon-carbon double bond. It undergoes addition reactions (e.g., catalytic hydrogenation to ethane) — so this is not the answer.