Final answer:
An indifference curve that slopes downward implies a trade-off relationship between two goods, where an increase in one must be counteracted by a decrease in the other to keep utility constant, signifying that both X and Y in the question are goods.
Step-by-step explanation:
If an indifference curve relating X and Y slopes downward, then it may be concluded that X and Y have a trade-off relationship where to maintain the same level of utility, an increase in one good must be offset by a decrease in the other good. An indifference curve represents all the combinations of two goods that provide the same satisfaction to a consumer. Points along the curve indicate that if a consumer has less of one good, they must have more of the other to be equally satisfied. Therefore, if the curve slopes downward, it suggests that both X and Y are indeed goods. This would not be true for bads, as an increase in a bad is not compensatory for a decrease in a good. The concept of an indifference curve is crucial in understanding consumer preferences and the marginal rate of substitution, which is the rate at which a consumer is ready to substitute one good for another while keeping their overall utility unchanged.