To answer this question, you need to understand hexadecimal numbers. Hexadecimal numbers are base-16 numbers that use the digits 0-9 and the letters A-F to represent values from 0 to 15.

Let's go through each option to understand why it is correct or incorrect:

Option A) 999 - This option is a decimal number because it uses the digits 0-9 only. Decimal numbers are base-10 numbers.

Option B) (hex)23 - This option is a hexadecimal number because it uses the prefix "(hex)" and the digits 0-9 and A-F.

Option C) 0x556 - This option is a hexadecimal number because it uses the prefix "0x" and the digits 0-9 and A-F.

Option D) 0x1F2 - This option is a hexadecimal number because it uses the prefix "0x" and the digits 0-9 and A-F.

The correct answer is B) (A) & (B). Option A (999) is a decimal number, and Option B ((hex)23) is a hexadecimal number. Therefore, they are not hexadecimal numbers.

So, the correct answer is B) (A) & (B).

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