The correct answer is C. Zero.
Income elasticity of demand is a measure of how responsive the demand for a good or service is to changes in income. It is calculated by dividing the percentage change in the quantity demanded of a good or service by the percentage change in income.
If the income elasticity of demand is zero, it means that the demand for a good or service does not change at all when income changes. This is because the good or service is considered to be a necessity. Buttons are a necessity, as they are used to fasten clothing. Therefore, the demand for buttons does not change when income changes.
Option A is incorrect because it is not possible for the income elasticity of demand to be infinity. This is because the demand for a good or service cannot be infinitely responsive to changes in income.
Option B is incorrect because it is not possible for the income elasticity of demand to be 1. This is because the demand for a good or service cannot be perfectly responsive to changes in income.
Option D is incorrect because it is not possible for the income elasticity of demand to be 5. This is because the demand for a good or service cannot be five times as responsive to changes in income as the demand for another good or service.