Answer:
When people are tired, they can't think very well.
Step-by-step explanation:
A pronoun is a part of speech that replaces a noun.
An antecedent on the other hand, is a word that comes before a pronoun or any other word.
There are some pronoun-antecedent agreements and the one that was corrected is the agreement by number.
"When someone is tired, they can't think very well."
There was no agreement by number here as "Someone" was singular" and the antecedent "they" was plural.
Therefore, it was changed to
"When people are tired, they can't think very well."
The singular "Someone" was changed to the plural "people" and the verb "is" was changed to "are"