Answer:
Among the given options the expression which is a radical equation is:

Explanation:
An equation in terms of a variable is said to be a radical equation if the variable is present inside a radical sign.
a)

This is not a radical equation since the variable x is not under the radical sign.
Hence, option: a is incorrect.
b)

This is a quadratic equation and not a radical equation.
Hence, option: b is incorrect.
c)

From the equation we see that a constant is present in a radical sign and not the variable.
Hence, option: c is incorrect.
d)

This is a radical equation as the variable is present in the radical sign.