Answer: The topic sentence
Explanation: If the text is properly written, the topic sentence should either be at the beginning of the paragraph/text, or the end, if it is a short text; for things like the heading or title; words in special print; repeated words; things reiterated by pronouns; or ideas being suggested.
In order to find theme look for what the story is trying to teach you. You could also look for the life story the author included.