39. Bitwise Rotation and Counting-II

Question.4

A developer uses the n & (n - 1) trick to check if a register has exactly one error flag set:

uint8_t err_flags = read_error_register();
if ((err_flags & (err_flags - 1)) == 0) {
   handle_single_error(err_flags);
} else {
   handle_multiple_errors(err_flags);
}

Under what condition does this code misclassify the error state?

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When err_flags has exactly 2 bits set
When err_flags is 0 — no errors are flagged, but the code treats it as a single error
When err_flags is 0xFF — all bits set
It never misclassifies — the trick works perfectly for all cases

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