Answer:
The topic sentence would most likely be the first sentence. (You didn't underline a sentence, so I'm giving you the straight answer.)
Step-by-step explanation:
The reasoning for that would be because the sentence is the only one that is introducing what is going to be happening in the paragraph. It basically tells you what is in the paragraph without the reasoning and the explaining.
All the other sentences are just supporting details/ideas to the main idea not the topic sentence.
If you had to underline anything it would be the first sentence for the topic sentence.
If you have any questions related to my answer to the problem, please do not be afraid to ask! I hope this helped. :)