Here are some quick tips for reaching this answer:
- You can remove the adjective clause (“…which was wrested by France
from the Genoese in 1768…”) because this is a nonessential clause, or unnecessary to the rest of the grammar.
- The adverbial time phrase (Over the past two decades…) gives you a clue for why A is the strongest choice. This time phrase is specific and connected to now (during the two decades before and including now). The present perfect (have seen) and present (sink) connect to the same time.