The correct answer is: C. an upward shift of the demand curve.
When income rises, people have more money to spend on goods and services. This includes oranges. As a result, the demand for oranges will increase, and the demand curve will shift to the right.
Option A is incorrect because it describes a movement along the demand curve, not a shift in the demand curve. A movement along the demand curve occurs when the price of the good changes, while other things remain unchanged. In this case, the price of oranges is not changing, so there is no reason for the demand curve to shift.
Option B is incorrect because it describes a movement along the supply curve, not a shift in the supply curve. A movement along the supply curve occurs when the price of the good changes, while other things remain unchanged. In this case, the price of oranges is not changing, so there is no reason for the supply curve to shift.
Option D is incorrect because it describes a decrease in demand, not an increase in demand. A decrease in demand occurs when the price of the good decreases, while other things remain unchanged. In this case, the price of oranges is not changing, so there is no reason for the demand to decrease.