Answer:
b. productive resources are limited.
Explanation:
Productive resources are mainly the resources which are used to make services and goods. Michael Emerson is a famous economist who stated that people's "want" is never ending. People seeks more and more from life and is never satisfied with what they 'have'. Now since there is a scarcity of productive resources, human want and desires grow more big and is never satisfied with the resources that they have.
Hence correct option is (b).