Phytophagous insects feed on :

The correct answer is Option (4) → Plant sap

Option (4) Plant sap — Correct. Phyto means plant, so phytophagous insects feed on plants. Of the options, only plant sap comes from a plant.

NCERT Extract:
Population Interactions – “… For plants, herbivores are the predators. Nearly 25 per cent of all insects are known to be phytophagous (feeding on plant sap and other parts of plants). The problem is particularly severe for plants because, unlike animals, they cannot run away from their predators.”