Phytophagous insects feed on :

Plant sap

The correct answer is Option (4) → Plant sap

Phytophagous insects are herbivorous insects that feed on plant materials such as leaves, stems, and plant sap. 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.

Other options, like excreta, eggs of frogs, or animal's blood, are not food sources for phytophagous insects, which specifically feed on plant matter.