This question seems to be incomplete. However, there is enough information to find the right answer.
Answer: a. Met / was reading / sat
We met each other one day while John was reading a book in the library and I sat down beside him.
Explanation:
Out of the options provided, only the chosen one correctly uses the verb tense for the missing words in the sentence: all the actions happened in the past.
Option c is incorrect because it uses the present tense (meet) and the past perfect (have sat) as if the act of sitting had happened before they met.
Option b. is incorrect because both actions, meeting, and seating, are in the past perfect tense, so there is an action that should happen after that that is missing.
Option d. is incorrect because the act of meeting happens in the future.
