If a medication is to be given to only the right ear, which abbreviation would best describe that?

The abbreviation that best describes administering medication to only the right ear is "d," which stands for "dextrus." In medical terminology, "dextrus" refers to the right side. Therefore, when a medication is specified to be given to the right ear, the use of this abbreviation clearly indicates that the treatment is localized specifically to that ear.

In contrast, the other options do not appropriately indicate administration to the right ear. For instance, "u" typically refers to "unit," which doesn't convey directional information. The abbreviation "a" can be interpreted as "auris," meaning ear but not specifying which side. Lastly, "o" generally stands for "oculus," meaning eye, which is not relevant when discussing the administration of medication to the ear. Thus, "d" is the most precise and accurate choice for indicating the right ear in this context.