Final answer:
The infective stage of canine heartworms is L3 larvae. The correct answer is c).
Step-by-step explanation:
The infective stage of canine heartworms is L3 larvae.
Adult heartworms live in the right ventricle of the heart in dogs and produce microfilariae. The microfilariae are then transmitted to a healthy dog through the bite of an infected mosquito. Once inside the dog, the microfilariae develop into L3 larvae in the subcutaneous tissues.
It is important to note that the other options mentioned in the question (microfilariae, adult worms, and L1 larvae) are also stages in the heartworm life cycle, but they are not the infective stage.