Which of the following abbreviations indicates a frequency of once daily?

The abbreviation that indicates a frequency of once daily is indeed “D” for "Every day." This term explicitly communicates that a medication should be administered once each day, which is a common practice in medication administration to maintain consistent levels of a drug in the patient’s system.

In contrast, the other options represent different frequencies: "b.i.d." stands for "bis in die," which means twice daily; "t.i.d." means "ter in die," indicating three times daily; and "q.o.d." means "quaque altera die," which refers to every other day. Thus, choosing "Every day" accurately reflects the intended frequency of administration as once daily.