If @ means +, # means × and % means −, what is 8 # 3 @ 6 % 4, following the usual order of operations?

The correct answer is B. 26

Replace the symbols: 8 × 3 + 6 − 4. Multiply first: 24, then add and subtract from left to right: 24 + 6 − 4 = 26. Forgetting the order of operations and working left to right would also give 26 here, but not in general, so always apply it.