Final answer:
In the sentence 'Marge might win the prize ________ Charlie might win the prize.', the conjunction 'or' should be used to show a choice or possibility between Marge and Charlie winning the prize.
Step-by-step explanation:
The question presented asks us to complete a sentence that needs a conjunction to connect two independent clauses: 'Marge might win the prize ________ Charlie might win the prize.
The conjunction required here is 'or' as it denotes a choice or possibility between two different outcomes.
Using 'or' in the sentence indicates that either Marge or Charlie might win the prize, but it's not certain who will.