Final answer:
The optimal time to administer a rectal drug for systemic effect is after a bowel movement, ensuring the rectum is empty for maximum drug absorption into the systemic circulation.
Step-by-step explanation:
The best time to administer a rectal drug for systemic effect is c. after a bowel movement. This is because the rectum should ideally be empty to ensure that the drug is absorbed properly into the systemic circulation. If the rectum contains feces, it may absorb some of the drug or impede the absorption process, resulting in a less effective dose reaching the systemic circulation. Additionally, having a bowel movement prior can help to avoid the expulsion of the rectal drug dosage form.