Hello!
The most accurate answer is
C. J’étais
This is because for one B (Was) and D (I have) make no sense at all in this statement. The statement is asking
When _____ in France, I visited several monuments.
Answer A) is very tricky because A has the same meaning as Answer C they both mean (I was)
But choice C is correct because of the past tense used in this sentence.
Glad to help! :)