As per the following grammar rule found in this GMAT article, the singular "make" should be used but it shows "makes" in the answer. This is a little bit confusing.
Rule: When acting as subjects of a sentence, gerunds and infinitives are always singular and require singular verbs.
Sentences:
Entertaining multiple goals (makes / make) a person’s life stressful.
To plan road trips to three different cities (involves / involve) the handling of many details.
Answer:
Entertaining multiple goals "makes" a person’s life
stressful.
“Entertaining multiple goals” is a gerund phrase which acts as the subject of the sentence ("singular").
To plan road trips to three different cities "involves" the handling of many details.
“To plan roads trips” is an infinitive phrase which acts as the subject of the sentence ("singular").