Which of the following is classified as an insect growth regulator?

Insect growth regulators (IGRs) are substances that disrupt the normal development of insects, typically affecting their growth processes or reproduction. Pyriproxyfen is indeed one of the notable IGRs that is often used in mosquito control, as it mimics juvenile hormones to prevent the maturation of larvae into adults. Methoprene is another well-known IGR that functions similarly, inhibiting the growth and maturation of juvenile insects by interfering with their hormonal balance.

When considering deltamethrin and malathion, these are classified as insecticides rather than growth regulators. Deltamethrin is a synthetic pyrethroid that works as a nerve agent to kill insects upon contact or ingestion, while malathion is an organophosphate that disrupts the nervous system of insects. Therefore, methoprene is the correct answer as it is explicitly designed to regulate Insect growth, preventing them from transitioning to the adult stage.