The section presents a question related to the identification of elements in the periodic table. It provides four answer options regarding what defines each element: 

A. Atomic mass (số khối)  
B. Number of protons (số proton)  
C. Number of electrons (số electron)  
D. All are incorrect (Tất cả đều sai)  

The key topic is the identification of elements in the periodic table, with a focus on the significance of protons, electrons, and atomic mass. The correct answer is related to the fundamental properties of elements.