Which one of the following statements is correct?

• A

  The radius $(R)$ of a nuclide of mass number $A$ is given by the equation $R=R_0(A)^{1 / 2} \quad\left(R_0=\right.$ constant)
• B

  ${ }_7 \mathrm{~N}^{15}$ and ${ }_8 \mathrm{O}^{16}$ are isobars
• C

  The end product nuclide in thorium ( $4 n)$ series is ${ }_{83} \mathrm{Bi}^{209}$
• Correct answer
  D

  ${ }_{20} \mathrm{Ca}^{40}$ has magic number of protons and magic number of neutrons

Hence, the correct answer is option: 'D'.