Which corrects the shift in tense in this sentence?
when they had been thinking about going to the keys, they did not know how warm it would be at that time of the year.
a.
think
b.
are thinking
c.
have thought
d.
thought

The  correct shift in tense in this sentence is:

C. have thought 

To correctly state the occurrence of events in their order, you have to use the present simple tense. In this sentence you are to emphasize the act of thinking at the time of speaking before the actual experience.