Distinguish short run and long run in macroeconomics.

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The main idea is how flexibility changes with time. In the short run, at least one input is fixed—usually the capital stock—so a firm can’t alter its plant size or equipment right away. It can vary labor and other variable inputs to adjust output, but the production capacity itself can’t be changed immediately. In the long run, all inputs are variable: a firm can expand or shrink its plant, buy new machinery, adopt new technology, and adjust the scale of operations. That combination—fixed inputs in the short run and fully adjustable inputs in the long run—best captures the standard distinction. The other statements either misstate which inputs are fixed in the short run or rely on price changes rather than the degree of input adjustability.