Which of the following notations represents an isotope of ${}_{25}{}^{54}\mathrm{Z}$?

${}_{25}{}^{54}\mathrm{Z}$ represents the isotope of ${}_{25}{}^{54}\mathrm{Z}$.

Isotopes are atoms of the same element having the same atomic number, but different mass numbers.

${}_{25}{}^{54}\mathrm{Z}$ has atomic number 25 and mass number 54. ${}_{25}{}^{55}\mathrm{Z}$ has the same atomic number 25, but a different mass number of 55.

Hence, ${}_{25}{}^{55}\mathrm{Z}$ is an isotope of ${}_{25}{}^{54}\mathrm{Z}$.