Hey there! I'm happy to help!
Let's see where the word tumultuous is used here:
Magellan named the ocean Pacifico (Pacific) which means peaceful because, compared to the tumultuous strait they had just been through, the ocean was amazingly calm.
We see that they named the ocean the Pacific Ocean because it was very peaceful and calm compared to a tumultuous strait. This shows us that tumultuous is probably the opposite of peaceful. It probably means crazy, hectic, rough, harsh, etc. After going through a crazy thunderstorm, a light sprinkle would seem very peaceful, which is similar to how Magellan probably feels here.
This definition matches best with answer A. rough and uncertain.
Have a wonderful day! :D