The section presents a question regarding the classification of iron (Sắt) in the periodic table. It provides four answer options related to different groups of elements: 

A. Alkali metals (Kim loại kiềm)  
B. Alkaline earth metals (Kim loại kiềm thổ)  
C. Transition metals (Kim loại chuyển tiếp)  
D. Nonmetals (Phi kim)  

The key topic is the classification of iron within the periodic table, specifically identifying which group it belongs to. The correct answer is C. Transition metals.