Final answer:
Fleas inject into subcutaneous tissues, making 'A. Subcutaneously' the correct answer. Oral administration of antimicrobials is convenient for treating systemic infections at home. Infected mosquito larvae are transmitted into a dog's subcutaneous tissues.
Step-by-step explanation:
Fleas penetrate the skin of their hosts and inject their saliva, which contains anticoagulants and other substances, into subcutaneous tissues. So, the correct answer to the question is A. Subcutaneously.
In terms of treating a systemic infection at home, an oral route of administration would be appropriate and convenient, so the answer is A. oral.
Furthermore, when an infected mosquito transmits larvae to a dog, the larvae are deposited in the dog's subcutaneous tissues. Over time, these larvae can move through various tissues, potentially including the bloodstream or exiting through the skin.