There are three isotopes of carbon differing with respect to:

a. neutrons
b. atomic number
c. nuclear charge
d. electron configuration
e. number of protons

a. neutrons

Isotopes differ in number of neutrons. Protons are the same. Electron configurations are the same. Nuclear charge is the same. Atomic number is the same (AND is the number of protons).

The correct answer is a. neutrons. The three isotopes of carbon, known as carbon-12 (C-12), carbon-13 (C-13), and carbon-14 (C-14), differ in the number of neutrons in their nuclei. Carbon-12 has 6 neutrons, carbon-13 has 7 neutrons, and carbon-14 has 8 neutrons. The number of protons and the atomic number for carbon remains constant at 6 for all isotopes.

The correct answer is (a) neutrons. Isotopes are different forms of an element that have the same number of protons but a different number of neutrons. Since the number of protons determines the atomic number and nuclear charge, and the arrangement of electrons determines the electron configuration, these options are not applicable for identifying isotopes. To determine the number of neutrons in an isotope of carbon, you would subtract the atomic number (which is 6 for carbon) from the mass number of the isotope.