**Core Concept**
Isotopes are atoms of the same chemical element that have the same number of protons (atomic number) but differ in the number of neutrons in their nuclei, resulting in different mass numbers. This fundamental concept is crucial in nuclear medicine, radiation therapy, and nuclear physics.

**Why the Correct Answer is Right**
Isotopes are characterized by their unique mass numbers, which are determined by the total number of protons and neutrons in the nucleus. Since the atomic number (number of protons) remains the same, the chemical properties of the element are preserved, but the physical properties, such as density and radioactivity, can vary significantly. For example, carbon-12, carbon-13, and carbon-14 are all isotopes of carbon with different mass numbers.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incomplete and does not provide any information about the substance in question.
* **Option B:** This option is not relevant to the concept of isotopes and does not address the question.
* **Option C:** This option is not applicable to the concept of isotopes, as it refers to a different scientific concept altogether.

**Clinical Pearl / High-Yield Fact**
Remember that isotopes are atoms of the same element with the same atomic number but different mass numbers, which can have significant implications in various fields, including medicine.

**Correct Answer:** D. Isotopes.