To solve this question, the user needs to have knowledge of number systems and the concept of place value.

The highest digit in any number system is determined by the base of that number system. The base represents the number of unique digits used in that system. For example, in the decimal number system (base 10), the unique digits are 0-9. Therefore, the highest digit in the decimal system is 9.

Now, let's go through each option and explain why it is right or wrong:

A. Zero: This option is incorrect. Zero is not considered a digit in any number system. It is used to represent the absence of a value, but it does not have a place value or contribute to the magnitude of a number.

B. Base - 1: This option is correct. The highest digit in any number system is one less than the base of that system. For example, in the binary number system (base 2), the highest digit is 1. In the decimal system (base 10), the highest digit is 9.

C. Base 1: This option is incorrect. Base 1 does not exist as a number system. A number system requires at least two unique digits to represent numbers. Base 1 would only have the digit 1, which is not enough to create a functional number system.

D. None of the above: This option is incorrect. As explained above, the highest digit in any number system is determined by the base of that system. Therefore, option D is not a valid response.

The Answer is: B. Base - 1