It is very rare to see green turtles off the coast of California; ________, during the summer they can be found among the rocks.
Which of the following words BEST completes the sentence above?

The two parts of the sentence are meant to contrast with each other, and only "however" expresses contrast. The sentence should read "It is very rare to see green turtles off the coast of California; however, during the summer they can be found among the rocks.".