The law of increasing opportunity costs exists because:

a. resources are not equally efficient in producing various goods.
b. the value of the dollar has diminished historically because of persistent inflation.
c. wage rates invariably rise as the economy approaches full employment.
d. consumers tend to value any good more highly when they have little of it.

The answer to this question is A: Resources are not  equally efficient in producing various goods

Explanation:

To understand the law of increasing opportunity cost. we must first understand the concept of opportunity cost.

Opportunity cost is the alternative forgone in other to satisfy other wants. This occur as a result of lack of sufficient resources to satisfy all wants therefore the less important wants has to be forgone so as to satisfy the more pressing needs.  

The law of increasing opportunity costs therefore states that as you increase production of one good, the opportunity cost to produce an additional good will increase.

The law of increasing opportunity cost can be better illustrated with production possibility boundary which shows the possible combinations of commodities that can be   produced in a given economy given the prevailing level of technology if all the available resources are efficiently utilized.

The PPC assumes that an economy can produce two goods (lets say cars and cassava ). If the country or economy focuses all its resources towards the production of cars there will be no resources available to produce cassava. on the other hand, if it choose to devote all resources towards the production of cassava, it will be able to produce 0 cars.

However, if the country choose to  produce both goods it means it will produce less of a particular goods in other to produce more of another goods.  

In a nutshell the concept of opportunity cost and production possibility curve exist due to limited availability of resources to produce all the goods or products needed by an individual, firm or country.