Answer:
B. Exploring the river, Ruth soon stumbled on a gigantic alligator.
Step-by-step explanation:
With modifiers, we use small changes in the sentence to make the words fit better together. In this sentence, we can see that this is not just an alligator that Ruth stumbled upon, but a gigantic one. It's a minor change to the sentence, but it makes it a better one. We can now visualize what Ruth came across, and it is a clear statement.
The other sentences just don't really seem that clear or sound very nice.
I hope this helps!
-No one