If a * b = a − b, what is the value of 5 * (2 * 1)?

Correct answer: 4

The governing concept is a defined binary operation together with evaluation of a nested expression. The rule a * b = a − b must be used whenever the symbol * appears; it does not mean ordinary multiplication. Because the expression contains parentheses, evaluate the inner operation first: 2 * 1 = 2 − 1 = 1. Substitute this result into the outer operation: 5 * (2 * 1) = 5 * 1 = 5 − 1 = 4. Therefore option B is correct. Option A is the result of stopping after the inner operation. Option C may arise from adding or mishandling the operands, and option D does not follow the defined rule. Correct bracket order and the given operation rule are both essential.