a
Explanation
Goods for which changes in consumption(or demand) are positively related to changes in income are said to be normal goods. If the given commodity is a normal good, an increase in income leads to an increase in its demand, while a decrease in income reduces the demand for the same.

As X is a normal good, a decrease in consumer's income would lead to a decline in its demand.

Verified Answer
The demand for good X would decrease.

b
Explanation
An inferior good is a good whose demand is inversely associated with the consumer's income, that is, the demand for inferior goods declines with a rise in consumer's income and vice versa.

As good YY is an inferior good, an increase in consumer's income would cause its demand to decrease.

Verified Answer
The demand for good YY would decrease.

c
Explanation
Substitute goods are those goods that can be used in place of another good providing similar satisfaction of a particular want, that is, the consumer perceives both goods as similar. For instance, tea and coffee are substitute goods.

An increase in the price of good YY (substitute good) would make its consumption costlier relative to good XX consequently, the demand for the former declines and the demand for the latter increases.

Verified Answer
The demand for good XX would increase.

d
Explanation
Inferior goods are defined as those, demand for whom is inversely associated with the level of income. As the income rises, people demand less of these goods and as the income declines, people demand a relatively higher quantity of them at each given price.

Consequently, good YY being an inferior good does not mean a low-quality product than good XX, as being inferior does not necessarily imply inferior quality. It simply defines the negative relationship between the income and the demand for good YY.

Verified Answer
No, good YY is not necessarily an inferior-quality product when compared to good XX.