Option 1: Isocost -> Represents combinations of inputs with the same total cost, not output level.
Option 2: Indifference curve -> Shows combinations giving equal utility in consumer theory, not production output.
Option 3: Isoquant -> Represents all input combinations producing the same output level.
Option 4: Budget line -> Shows affordable combinations for consumers, not related to production output.
Hence, Option 3: Isoquant -> An isoquant is a curve in production theory that shows all possible combinations of two inputs (such as labor and capital) that produce the same level of output. The term comes from 'iso' meaning equal and 'quant' referring to quantity. For example, a firm might produce 100 units using either 10 workers and 5 machines, or 8 workers and 7 machines - both combinations lie on the same isoquant. This concept is analogous to indifference curves in consumer theory, but applies to production rather than consumption. -> correct