Answer:
b. each person must pay for exactly what he or she receives.
Explanation:
The expression "There's no such thing as a free lunch" means that each person must pay for exactly what he or she receives.
This phrase is a common dialogue by the people to suggest that there is no such thing as any free stuff. If a person wants anything, he/she has to pay or give something in return for the thing s/he's going to get. It doesn't necessarily mean the literal 'lunch'. It's just a common way of referring to the overall scheme of things that nothing is ever free in this world.