Choose the option opposite in meaning to the word(s) or phrase that is underline...

This question is asking you to choose the option that means the opposite of the word or phrase that is underlined. The underlined word in this question is "diurnal". "Diurnal" means happening during the day or active during the daytime. So, you need to choose the option that means the opposite of that.

The options are: an annual, a weekly, a nightly, and a short. You should choose the option that is the opposite of "diurnal". The opposite of "diurnal" is something that happens during the night or is active during the night time.

Option A is "an annual". This means something that happens once a year. This is not the opposite of "diurnal", so it is not the correct answer.

Option B is "a weekly". This means something that happens once a week. This is also not the opposite of "diurnal", so it is not the correct answer.

Option C is "a nightly". This means something that happens during the night or is active during the night time. This is the opposite of "diurnal", so it is the correct answer.

Option D is "a short". This does not relate to the meaning of "diurnal", so it is not the correct answer.

Therefore, the correct answer to this question is Option C: a nightly.