Final answer:
The sentence with proper punctuation is: Jason, Marcus, and John all play on the varsity basketball team.
Step-by-step explanation:
The sentence with proper punctuation is: Jason, Marcus, and John all play on the varsity basketball team.
This sentence uses the comma to separate the names of the players and the phrase about their activity.
The incorrect options either use a comma before 'and' or place a comma after 'and,' both of which are incorrect punctuation.
Learn more about Proper punctuation in English grammar.