The highest common factor of 0 and 6 is:

The correct answer is (a).

The highest common factor (HCF) of two numbers is the largest number that is a factor of both numbers. In other words, it is the largest number that can be divided evenly by both numbers.

0 is not a factor of any number, so the HCF of 0 and 6 is 0.

(b) is incorrect because 6 is not a factor of 0.

(c) is incorrect because 3 is not the HCF of 0 and 6.

(d) is incorrect because the HCF of 0 and 6 is not undefined.