I.  The marginal rate of substitution is the maximum amount of another good that a person is willing to give up to obtain one additional unit of the good. (True)

II. Two goods are perfect complements when the marginal rate of substitution of one good for the other is constant. (False, two goods are perfect substitutes when the marginal rate of substitution of one good for the other is constant.)

III. Two goods with right-angle indifference curves are perfect substitutes such as left and right shoes. (False, two goods with right-angle indifference curves are perfect complements such as left and right shoes.) 

The answer is A.