Which of the following is NOT a valid octal number?

The correct answer is B, 128. The octal system has base 8, so it uses only the eight digits from 0 to 7. The general rule is that the largest digit of any number system is one less than its base, which means the digits 8 and 9 can never appear in an octal number. The number 128 contains the digit 8, so it cannot be an octal number, although it is a perfectly valid decimal or hexadecimal number. Option A is wrong as a choice because 377 uses only 3 and 7, both of which are allowed. Option C is wrong because 745 uses 7, 4 and 5, all below 8. Option D is wrong because 66 uses only the digit 6. Apply the same test to binary, where only 0 and 1 are allowed, and to hexadecimal, where 0 to 9 and A to F are allowed.