The correct answers, which are compounds, are B, C, and D.
The compounds are substances composed of two or more different elements that are chemically bonded together.
B. H2O is water, a compound made of two hydrogen atoms bonded to one oxygen atom.
C. CuFeS2 is chalcopyrite, a compound composed of copper, iron, and sulfur.
D. KCl is potassium chloride, a compound made of potassium and chlorine.
The other options are not compounds:
A. Cl likely refers to chlorine, which is an element, not a compound. It would be a compound if it were Cl2, which is diatomic chlorine, but as written, it is not correct.
E. Mg is magnesium, which is also an element, not a compound.