Degree of zero polynomial is
Not define.
As We know that
The highest exponents of a variable is known as the Degree of a polynomial.
The degree of a term is the sum of the exponents of variables that appear in it and is a non-negative integer.
Also we know that, zero polynomials can be as, a polynomial with coefficient equal to zero.
Any polynomial with all the variables that have their coefficients equal to zero is called a zero polynomial.
For example - In the zero polynomial
The degree of the zero polynomial is not defined.
Hence Correct option is (d)