An "anti-knock agent" for petrol (or gasoline) is a chemical additive that is added to fuel to prevent engine knocking, which is a knocking sound produced by premature ignition in the combustion chamber, and essentially increases the fuel's octane rating by delaying ignition.

Option A - 2,3- dimethylpentane

Option B - 3-methylhexane

Option C - 3-methylhexane

Option D - 2,2-dimethylpentane

Both 2,3-dimethylpentane and 2,2-dimethylpentane are branched-chain structures. However, 2,2-dimethylpentane has a more compact, highly branched structure, which is generally associated with higher octane ratings and better anti-knock properties.

In contrast, 2,3-dimethylpentane has a slightly less branched structure, which may result in slightly lower octane ratings. So, while both compounds are branched, the degree of branching and the resulting octane rating can differ.