A. Polymeric compounds refer to compounds that have a relative molecular weight greater than 10,000 and are primarily composed of many atoms or atomic groups linked by covalent bonds. Proteins are organic polymer compounds, but oils are not, so A is false;
B． Solar energy is clean energy and its application is conducive to environmental protection and energy saving. Crystalline silicon can be used to make solar cells, which is conducive to environmental protection and energy saving. Silica is a fiber optic material, so B is wrong. ;
C. Na, C and S can all react directly with oxygen. Sodium reacts with oxygen to form sodium oxide or sodium peroxide, carbon reacts with oxygen to form carbon monoxide or carbon dioxide, and sulfur reacts with oxygen to form unique ment of sulfur dioxide, therefore C. is false;
D. Sucrose and maltose have the same molecular formula but different structures and are isomers of each other, so D is correct;
So choose D.