In economic theory, all of the listed variables can be relevant when exploring relationships between inputs and outputs. However, if you are considering a variable that specifically deals with the relationship between inputs and outputs, you might want to focus on the Marginal Product.
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Elasticity of Supply measures how responsive the quantity supplied of a good is to a change in its price, but it's more about supplier behavior rather than a direct input-output relationship.
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Marginal Product refers to the additional output produced by adding one more unit of an input while keeping other inputs constant. This is a direct input-output relationship.
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Marginal Propensity to Consume (MPC) and Marginal Propensity to Save (MPS) relate to how changes in income affect consumption and savings, but they are less directly connected to the input-output relationship in production.
So, the most applicable variable for examining input-output relationships specifically is Marginal Product.