The radius of a nucleus varies with its mass number (A) as

directly proportional to $\sqrt[3]{A}$

The correct answer is Option (3) → directly proportional to $\sqrt[3]{A}$

The radius of a nucleus is given by:

$R = R_{0} A^{\frac{1}{3}}$

Hence, it is directly proportional to $A^{\frac{1}{3}}$.

Answer: directly proportional to $(A)^{\frac{1}{3}}$