An element may have several isotopes that contain different numbers of but the same number of a) quarks, mesons b) neutrons, protons c) tachyons, baryons d) electrons, protons

An element may have several isotopes that contain different numbers of NEUTRONS but the same number of PROTONS.

Explanation:

Isotopes are defined as the group of such atoms which will have same number of protons but different number of neutrons

So this is basically same atoms but with different number of neutrons

So here we can sat

above are three isotopes of hydrogen

similarly we have many more examples of isotopes in which the atoms are of same type but with different number of neutrons

so here correct answer will be

An element may have several isotopes that contain different numbers of NEUTRONS but the same number of PROTONS.

the answer would be a