The section presents a question related to the characteristics of an atom of a chemical element. It lists four options regarding what is a defining feature of an atom, specifically focusing on the number of protons, neutrons, and electrons. The correct answer is indicated to be a combination of the number of protons and neutrons. 

Key Topics:
- Characteristics of atoms
- Protons, neutrons, and electrons

Entities:
- Problem number: 1
- Problem ID: KHTN7C1B1_L2_1
- Options A, B, C, and D related to atomic structure.