The relationship between an increase in variable input and output when all other inputs are held constant is referred to as.

Marginal Product.

The correct answer is Option (3) → Marginal Product.

Marginal Product (MP) refers to the additional output produced when one more unit of a variable input (like labor) is added, while all other inputs are held constant. It shows the change in total product resulting from a change in the variable input, and is a key concept in the law of variable proportions.