Final answer:
Lufenuron is the ingredient that controls flea populations by preventing flea eggs from hatching, due to its ability to inhibit the synthesis of chitin.
Step-by-step explanation:
The ingredient that controls flea populations by breaking the life cycle at the egg stage is Lufenuron. Lufenuron is a chitin inhibitor, which means it interferes with the synthesis of chitin, a material that makes up an insect's exoskeleton. By disrupting the production of chitin, lufenuron prevents flea eggs from hatching, effectively controlling the flea population. This method of pest control is different from substances like Selamectin, Fipronil, and Imidacloprid, which work by affecting the nervous system of the fleas.