Isotopes of an element differ in:
a. the number of protons
b. number of neurons
c. number of electrons
d. atomic number

The correct answer is B. Isotopes of an element differ in the number of neutrons.

Explanation:

Isotopes are atoms of the same chemical element, and therefore with the same number of protons, in which the number of neutrons in the atom core differs. According to current models, an atom consists of an atomic nucleus surrounded by a cloud of electrons. The core consists of a number of protons and a number of neutrons. The number of protons determines the atomic number of the element and also the chemical properties. The number of neutrons determines, among other things, the stability of the nucleus and, together with the number of protons, the atomic mass. Two atoms with the same number of protons but a different number of neutrons are called two isotopes of the same element.

the endomembrane system i think