What structural feature is different in isotopes of a particular element

atomic mass and number of neutrons

Explanation:

Isotopes are forms of same element having same number of protons.  

As atomic number is same as number of protons, therefore, isotopes of an element have same atomic number also.

Isotopes differ from each other in having different number of neutrons.  

Mass number is equal to no. of protons and no. of neutrons. As in isotopes no. of neutrons are different, therefore mass number will also be different.

Therefore, isotopes have different atomic mass and number of neutrons.

the answer would be a