(B) T5
Due to biological magnification, harmful chemicals accumulate progressively at each trophic level. T5, being the highest trophic level, has the maximum concentration of these chemicals.
Biological magnification means non-degradable chemicals (like pesticides) get more concentrated as they move up the food chain. The apex consumer (T5) accumulates the highest amount. NCERT explicitly states: "As human beings occupy the top level in any food chain, the maximum concentration of these chemicals get accumulated." Always remember: highest trophic level = maximum chemical concentration.