Identify free radical inhibitor from following. (A) (B) O_2 (C) (D)...

Free radical inhibitors are substances that react with reactive free radicals to form relatively stable and unreactive radicals, thereby breaking the chain reaction.

(A) p-Benzoquinone acts as a free radical inhibitor. It reacts with a free radical to form a highly resonance-stabilized and unreactive aryloxy radical, which stops the chain reaction.

(B) Oxygen ($\text{O}_2$) is a diradical and acts as a free radical inhibitor. It reacts with highly reactive alkyl radicals to form peroxy radicals ($\text{R-O-O}^{\bullet}$), which are much less reactive and thus slow down or inhibit the free radical chain reaction.

(C) Benzene is not a free radical inhibitor.

(D) $\text{CrO}_3$ is an oxidizing agent and does not act as a free radical inhibitor.

Therefore, only (A) and (B) are free radical inhibitors.

Answer: (A) and (B) Only