Which of the following is the correct abbreviation to denote both ears?

The abbreviation that denotes both ears is "u," which stands for the Latin term "auris utra," meaning "both ears." This notation is commonly used in medical prescriptions and documentation to indicate that a treatment or medication should be applied or assessed in both ears.

In contrast, "o" typically refers to the left ear (from the Latin "auris sinistra"), while "d" indicates the right ear (from the Latin "auris dextra"). The abbreviation "qpm" means "quater in die" or "four times a day," which is unrelated to the context of referring to ears. Therefore, "u" is the appropriate choice when indicating that something pertains to both ears.