B is formally an octal number (preceded by 0), but the octal number should only be 0~7, and there will be no numbers such as 8 and 9.
C is an octal long integer.
D looks like a hexadecimal integer at first glance, but when using hexadecimal numbers, you must add the prefix 0x, otherwise the 0 1ff compiler will not recognize it.
However, what you are looking for is an integer variable. I'm sorry to tell you that there are no "variables", only expressions (a) and constants (b, c, d- legal and illegal).