First and foremost, we must work out how we are going to get there. A. think B. plan C. write D. che

First and foremost, we must work out how we are going to get there. A. think B. plan C. write D. check

B. plan

Explanation:
The phrase “work out” in this context means to figure out, develop, or decide the details of something.
“Plan how we are going to get there” is the natural match.
The other verbs are less suitable:

think could be possible but is less complete than “plan” (planning involves thinking, but specifically means arranging details).

write and check don’t match the meaning of “work out” here.