Consider the definition of a rational number.
A rational number is the one that can be written in the form of p/q, where p and q are integers and q ≠ 0.
Zero can be written as 0/1,0/2,0/3,0/4,0/5,..... .
So, we arrive at the conclusion that 0 can be written in the form of p/q, where q is any integer.
Therefore, zero is a rational number.