_______ of an input is defined as the change in output per unit of change in the input when all other inputs are remain constant.

Marginal Product.

The correct answer is Option (1) → Marginal Product.

Marginal Product (MP) of an input is defined as the change in output resulting from a one-unit change in that input, while keeping all other inputs constant.

MP= Δ Output / Δ Input 

• Average Product: The total output divided by the total number of units of an input used.

• Total Product: The total quantity of output produced with a given set of inputs.

• Returns to Scale: A long-run concept that describes how output changes when all inputs are increased proportionally.