Variable inputs are defined as any resource that:

• A. is only used in the long run
• B. can be changed in quantity in the short run to alter output
• C. remains fixed regardless of the level of production
• D. cannot be substituted for other inputs

Answer: (B)

Verified step by step guidance

1. Understand the definition of variable inputs in microeconomics: Variable inputs are resources or factors of production whose quantity can be changed in the short run to affect the level of output.
2. Recall that in the short run, some inputs are fixed (cannot be changed), while variable inputs can be adjusted to increase or decrease production.
3. Analyze each option given: - 'Is only used in the long run' is incorrect because variable inputs are relevant in the short run. - 'Can be changed in quantity in the short run to alter output' aligns with the definition of variable inputs. - 'Remains fixed regardless of the level of production' describes fixed inputs, not variable inputs. - 'Cannot be substituted for other inputs' is unrelated to the definition of variable inputs.
4. Conclude that the correct understanding of variable inputs is that they can be changed in quantity in the short run to alter output.
5. Summarize that variable inputs are flexible resources in the short run, such as labor or raw materials, which firms adjust to meet production needs.