Which country had the most powerful navy in the sixteenth century? a. england b. france c. spain d. netherlands

I believe this is Spain.
Spain was at the height of its strength, its empire was the largest among all European nations and maintained both the European waters as well as those in the West Indies. Few countries were a threat for this navy. Even with the Armada disaster, I think it's fair to say that Spain was the dominant naval power in this century.

The answer is option a. boundary line drawn by pope alexander iv in 1493.