b) The price of the good is the right answer. A consumer moves along the demand curve as a result of a change in the price of the good.

The other are not correct because

a) price of a complement good causes demand curve to shift, e.g., a consumer will demand more of the good if a complement becomes cheaper and it is shown by a rightward shift in demand curve

c) price of a substitute good causes demand curve to shift, e.g., a consumer will demand more of the good if a substiture becomes more expensive and it is shown by a rightward shift in demand curve

d) demand curve shift rightwards if a buyer's income increases (this is of course true for a normal good)