Identify the pairs which are not of isotopes?

(a) ${}_6{}^{12}\mathrm{X}\text{ and }{}_6{}^{13}\mathrm{Y}$ have same atomic number but different mass number.

(b) ${}_{17}{}^{35}\mathrm{X}\text{ and }{}_{17}{}^{37}\mathrm{Y}$ have same atomic number but different mass number.

Both these pairs are isotopes to each other.

c) ${}_6{}^{14}\mathrm{X}\text{ and }{}_7{}^{14}\mathrm{Y}$ have different atomic number but same mass number.

(d) ${}_4{}^8\mathrm{X}\text{ and }{}_5{}^8\mathrm{Y}$ have different atomic number but same mass number.

Both these pairs are isobars to each other.