Questions: The problem makes it difficult to determine the true demand for a good.

The answer is A: free-rider.

Explanation for each option:

A. Free-rider: The free-rider problem occurs when individuals can benefit from a good or service without paying for it, which makes it difficult to determine the true demand. This is because the good is non-excludable, meaning people cannot be prevented from using it, and non-rivalrous, meaning one person's use does not reduce availability to others. Public goods, like national defense or public parks, often face this issue.

B. Pricing: While pricing can influence demand, it does not inherently make it difficult to determine the true demand for a good. Pricing strategies can affect consumer behavior, but they are tools used to gauge and respond to demand rather than obscuring it.

C. Rivalry: Rivalry refers to the characteristic of a good where one person's consumption reduces the amount available for others. This is typical of private goods. While rivalry affects how goods are consumed, it does not inherently make it difficult to determine demand. In fact, rivalry can make demand more apparent as scarcity becomes a factor.