Answer: $1500
Explanation: As per the theory of microeconomics, opportunity cost is the cost of loosing the profits from the best alternative that was not selected. In simple words, it is the real cost of output foregone.
Usually opportunity cost is used to describe relationship between scarcity and choice.
In the given case Carlos was earning interest on savings but rather than that he invested it in the business. Therefore :-
opportunity cost = $50,000 * 3% = $1500