Right option is (c) The manner in which a drug is taken
To elaborate: Drug regimen is defined as the manner in which a drug is taken. For some of the drugs such as hypnotics, antiemetics, analgesics, etc., a single dose is enough to provide its effective treatment on the patient. But for some of the diseases such as cholera, typhoid, the dosage of the drug has to be calculated and given for a longer amount of time. In these cases, drugs are to be taken on a repitative basis over a period of time.