The Law of Variable Proportions assumes that:

Correct option is B

The Law of Variable Proportions applies to the short run, where one input is variable (like labor), and others (like land or capital) are fixed. It examines the change in output as the variable input is increased, holding others constant.
Explanation of options: (a) ✘ In the long run, all inputs are variable, but this law is about the short run. (b) ✔ Correct — the law assumes only one input varies while others remain fixed in the short run. (c) ✘ This contradicts economic theory — in the long run, inputs are never fixed. (d) ✘ All inputs fixed in the short run would mean no production change — not realistic.