The highest DDT concentration in aquatic food chain shall occur in:

This is due to biomagnification, where the concentration of a non-biodegradable toxicant increases at successive trophic levels. In an aquatic food chain (Phytoplankton $\to$ Zooplankton $\to$ Small fish $\to$ Large fish $\to$ Fish-eating birds), the seagull (top consumer) will have the highest concentration.