Humans are at the highest trophic level in a food chain. During bio-magnification, the concentration of harmful chemicals (like DDT or pesticides) increases at each successive trophic level. Since humans occupy the top of the food chain, the maximum concentration of these toxic substances accumulates in their bodies, making them the most adversely affected.
The key concept here is that bio-magnification means the concentration of non-biodegradable chemicals increases as you move up the food chain. Examiners expect you to mention: (1) humans are at the highest/top trophic level, and (2) maximum concentration of toxins therefore accumulates in humans. This is a 2-mark question, so two clear points suffice.