Which of the following is not isotope of carbon

Isotopes mean the species of an element have same atomic number and different mass number.

Isotopes of carbon; ${}_6{}^{12}\mathrm{C}, {}_6{}^{13}\mathrm{C}, {}_6{}^{14}\mathrm{C}$

Therefore, the nuclide tritium$\left({}_1{}^3\mathrm{H}\right)$is an isotope of hydrogen but not for carbon.