The correct answer is: C. Relative prices of the commodities.
The substitution effect is the economic concept that, when the relative price of a good changes, consumers will substitute away from the good that has become relatively more expensive and towards the good that has become relatively cheaper.
For example, if the price of apples goes up relative to the price of oranges, consumers will tend to buy fewer apples and more oranges. This is because apples have become relatively more expensive, so consumers will substitute away from apples and towards oranges.
The substitution effect is a key concept in economics, and it is used to explain a wide range of economic phenomena. For example, it is used to explain why consumers buy more of a good when its price goes down, and why they buy less of a good when its price goes up.
The substitution effect is also used to explain why firms produce more of a good when its price goes up, and why they produce less of a good when its price goes down.
The substitution effect is a powerful tool for understanding economic behavior, and it is used by economists to make predictions about how consumers and firms will behave in response to changes in prices.
The other options are incorrect because they do not explain the substitution effect.
Option A is incorrect because the income of consumers does not affect the substitution effect. The substitution effect is a purely relative price effect, and it is not affected by the income of consumers.
Option B is incorrect because the price of a single good does not affect the substitution effect. The substitution effect is a relative price effect, and it is not affected by the price of a single good.
Option D is incorrect because it is a combination of the incorrect options A and B. The substitution effect is not affected by the income of consumers or the price of a single good.