Which of the following are isotopes?

a U-235, Br-81

b Mg-12, Mn-55

c Cl-35, Cl-37

d H-1, H-1

C) Cl-35 & Cl-37

Explanation:

Isotope: a form of an element that differs only in the number of neutrons in the nucleus.

A & B are wrong because U and Br are two different elements, so are Mg and Mn.

D is a trick question because H-1 is the same as H-1, therefore they can't be isotopes.