In a digital representation of voltages using an 8-bit binary code, how many values can be defined?

ANS:D - 256

That seems to be quite the thing.

That is, if there are 8 bits in the signal and there are 2 possible representations of voltage levels in a binary number system. Then every bit has the possibility of being represented by any of those 2 numbers.

Hence, 2*2*2*2*2*2*2*2 = 2^8 = 256.

That was some explanation.