Answer: option(a) is correct.
Explanation:
Diminishing marginal returns also known as diminishing returns. It states that the additional input or factor of production that is used for the production of certain products and services, results in smaller increase in output. The output increases but at an decreasing rate. This is due to the efficiency or productivity of the additional factor of production. This will results in higher marginal cost because of the lower productivity from the additional input. So, marginal cost must be increasing.