Humans are at the top of the food chain (highest trophic level), so harmful chemicals accumulate in their bodies in the highest concentrations through biological magnification.
Ordinary washing cannot remove these chemicals because they enter the inner tissues of fruits and vegetables through absorption from the soil and water, and cannot be washed off from the surface.
Examiners look for two distinct points: (1) humans are at the top trophic level — so maximum concentration accumulates in them, and (2) the chemicals are absorbed internally into plant tissues, not just present on the surface — so surface washing is ineffective. Note: the source passages do not directly cover this topic, so the answer is based on standard CBSE Chapter 15 (Our Environment) content on biological magnification.