Final answer:
Lufenuron is the active ingredient that disrupts flea life cycles by preventing eggs from maturing into larvae.
Step-by-step explanation:
The active ingredient that controls flea populations by breaking the life cycle at the egg stage is Lufenuron. Lufenuron is classified as a chitin inhibitor, a type of compound that interferes with the synthesis of chitin, the main component of the insect exoskeleton. By inhibiting chitin production, Lufenuron prevents flea eggs from developing into larvae, effectively breaking the life cycle of fleas at the egg stage. The other options provided, such as Selamectin, Fipronil, and Imidacloprid, have different modes of action in controlling insect populations.