Answer: c. rightward shift of a demand curve.
Explanation:
When there is movement along the demand curve, this is due to a change in the price of the good.
However, an increase in demand is noted by a rightward shift in the Demand curve. This is to signify that the demand has changed even though the price had remained the same. This shift is meant to signify that something else apart from price has caused an increase in demand such as an increase in income. After the shift, the price will have to change to reflect a new Equilibrium which will be the new intersection point with the Supply Curve.
I have attached a graph showing what happens when Quantity Demand increases.
