Here are three things you could do if you do not go swimming with a friend: study for an upcoming test (you value this at $10), read a good novel (you value this at $18), or go in to work (you could earn $16 during the time you swim with your friend). The opportunity cost of going swimming is
a.$16, because this is the only alternative of the three for which you actually receive a monetary payment.
b.$10, because this is the lowest valued alternative forfeited.
c.$18, because this is the highest valued alternative forfeited.
d.$44, because this is the total dollar sum of the alternatives forfeited.