Final answer:
The Law of Variable Proportion states that output increases at a decreasing rate with each additional unit of input.
Step-by-step explanation:
The Law of Variable Proportion in economics states that as the quantity of a single input factor is increased while keeping all other inputs constant, the output will initially increase, but eventually increase at a decreasing rate. This means that option b) output increases at a decreasing rate with each additional unit of input accurately describes the relationship between input factors and output.