Answer
B) a different number of neutrons in their nuclei.
Explanation
Isotopes are members of a family of an element that all have the same number of protons but different numbers of neutrons. The number of protons in a nucleus determines the element's atomic number on the Periodic Table. For example, sodium has eleven protons and is atomic number 11.
Hence, isotopes have a different number of neutrons in their nuclei.
The correct answer is option B) a different number of neutrons in their nuclei.
