Sentence Correction

There are 2 splits in this question. One is the has/have (and are) split. I understand this. As the subject ('period') of the verb is singular, the verb has to be singular. Hence, it should be 'has'.

I'm confused about the second split which is towards the end of the underlined text: is/are. This example was taken up in a 'Thursdays with Ron' session wherein, he explained that when there is a list (in this case, 'use', 'reason' and 'meaning'), the entire list should be considered as one plural noun. Hence, the verb should be 'are', making the correct answer A. But, the answer is D. Pls tell me the correct rule.

TIA.