Answer: B) adult worms
Explanation: The drug melarsomine is used in the treatment of canine heartworm disease and is the only product available to kill the disease by specifically targeting and killing adult heartworms.
Melarsomine is administered by injection into the lower back muscles and works by causing damage to the adult heartworms, leading to their death. It is not effective against microfilaria, which are the immature forms or "babies" of the heartworms, or the vectors, which are the mosquitoes that transmit the disease. While it will not kill the microfilaria, it may reduce the number of microfilaria produced by the adult worms.
It is important to note that the treatment of heartworm disease should be done under the supervision of a veterinarian, as there are specific protocols and precautions that need to be followed to ensure the safety and effectiveness of the treatment.