Fill in the blank with the most appropriate word.
He ____ difficulty on the last test he took.

In the given sentence "He had difficulty on the last test he took," the most appropriate word to fill in the blank is "had."

The verb "had" is in the past tense and is used to indicate an action that occurred in the past. In this context, it signifies that he experienced difficulty on the last test he took, which is now completed and in the past.

The other options do not use the correct verb form for the given sentence. Option B, "has," is in the present tense and does not match the past timeframe mentioned in the sentence. Option C, "is having," is in the present continuous tense, which does not fit the context of describing a past event. Option D, "will have," is in the future tense, which is also not appropriate for describing a past action.

Therefore, the correct answer is "A. had."