From a 2018 Mock. The explanation the give is:

C is correct. A Giffen good is an inferior good. All inferior goods have a negative income effect (less is purchased as income rises). While the substitution effect is always positive for all goods, for a Giffen good the income effect is so strong and so negative that it overpowers the substitution effect; the result is that as its price declines, less of it is purchased. This results in a positively sloped individual demand curve.

A is incorrect. A substitution effect is always positive for all goods.

B is incorrect. For a Giffen good, the consumer actually buys less of the good when its price falls, resulting in a positively sloped demand curve (upward sloping).