​The law of diminishing marginal productivity states that

A) ​As you expand output,your marginal productivity eventually increases
B) As you expand output,your marginal productivity eventually declines
C) As you expand output,the total product eventually increases
D) ​None of the above

The law of diminishing marginal productivity states that as you continue to add additional units of a variable input to a fixed input, holding all other inputs constant, eventually the marginal productivity of the variable input will begin to decline. Therefore, option B is the correct answer. Option A is incorrect because marginal productivity eventually declines, not increases. Option C is incorrect because total product may or may not increase as output expands and it is not necessarily related to the law of diminishing marginal productivity. Option D is incorrect because option B is the correct answer.