The correct answer is D. downwards to the right.
An inferior good is a good whose demand decreases as income increases. This is because as people become wealthier, they tend to substitute away from inferior goods and towards more luxurious goods.
The income demand curve for an inferior good is therefore downward-sloping. This means that as income increases, the quantity demanded of the good decreases.
Option A is incorrect because a horizontal income demand curve would mean that the quantity demanded of the good is not affected by income. This is not the case for an inferior good.
Option B is incorrect because a vertical income demand curve would mean that the quantity demanded of the good is always the same, regardless of income. This is also not the case for an inferior good.
Option C is incorrect because an upward-sloping income demand curve would mean that the quantity demanded of the good increases as income increases. This is not the case for an inferior good.