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

Option 1 -> Average Product measures output per unit of variable input, not the change in output from adding more input.

Option 2 -> Total Product represents the total output produced, not the incremental relationship between input and output changes.

Option 3 -> Marginal Product specifically measures the additional output generated when one more unit of variable input is added, holding all other inputs constant.

Option 4 -> Physical Product is a general term that can encompass various product measures, but doesn't specifically define the incremental input-output relationship.

Hence, Option 3: Marginal Product -> Marginal Product (MP) is defined as the change in total output resulting from employing one additional unit of a variable input while keeping all other inputs fixed. This concept is fundamental in production theory and helps firms determine optimal input levels. The law of diminishing marginal returns states that MP initially increases, reaches a maximum, and then declines as more variable input is added to fixed inputs. -> correct