The correct answer is: All of these.
The demand curve slopes downward to the right because of the substitution effect and the income effect.
The substitution effect occurs when a change in the price of a good causes consumers to substitute other goods for it. For example, if the price of coffee increases, consumers may substitute tea or other beverages for coffee.
The income effect occurs when a change in the price of a good causes consumers’ real income to change. For example, if the price of coffee decreases, consumers will have more money to spend on other goods, which will increase the demand for those goods.
A change in the number of buyers can also cause the demand curve to shift. For example, if the population increases, the demand for goods and services will increase, which will shift the demand curve to the right.
In conclusion, the demand curve slopes downward to the right because of the substitution effect, the income effect, and a change in the number of buyers.