Answer:
Option (A) is correct.
Step-by-step explanation:
The marginal cost is also defined as the opportunity cost.
Opportunity cost refers to the value or quantity of one good that must be foregone to produce one extra unit of other good. Here, the opportunity cost of producing potatoes is the number of oranges.
If Monday Island wants to increase the production of potatoes then it must sacrifices some of the units of oranges.