Explanation

The sentence "One of my best ____ getting a Ph.D. degree today" requires careful attention to grammatical agreement and structure. The phrase "one of my best" indicates that we're talking about a singular subject that is part of a larger group. Here, "best friends" implies that the subject is counted as one individual in a group of friends.

The correct phrase to fill in the blank is "friends is." Although "friends" is plural, the use of "one of" signifies that we are focusing on a singular friend from a larger cohort of friends. Therefore, we treat it as singular when it comes to the verb agreement.

Option C, “friends is," fits this structure properly because "friends" serves as the object of the preposition "of," and the actual subject is "one," which is singular. This is crucial for correct subject-verb agreement, insisting that we use "is" instead of "are." Other options either use improper singular/plural agreement or create awkward phrases, which is why C is the best choice for completing the sentence correctly.