What is the degree of zero polynomial ?
A. 0
B. 1
C. Any natural number
D. Not defined

Hint – In order to solve this problem we need to know the concept that The degree of a polynomial is the highest degree of its monomials (individual terms) with non-zero coefficients. Using this will take you to the right answer.
Complete step-by-step answer:
As we know that : The degree of a term is the sum of the exponents of the variables that appear in it, and thus is a non-negative integer.
By this definition, the degree of a non-zero constant is zero and that of a zero polynomial is not-defined.
Hence it is not defined.
So, the correct option is D.
Note – To solve this problem we should use the key concept that, like any constant value, the value 0 can be considered as a (constant) polynomial, called the zero polynomial. It has no nonzero terms, and so, strictly speaking, it has no degree either. As such, its degree is undefined. Using this will take you to the right answer.