Which of the following represent elements and which represent compounds?

A. HC1

B. CO

C. Na

D. Ca

A and B are compounds

C and D are elements

In Chemistry, an element is shown by a single uppercase letter, sometimes followed by a lowercase letter.

In this case, Na is Sodium, and Ca is Calcium.

A and B have two uppercase letters, indicating that they are not an element, but a combination of elements, therefore, they are compounds (Hydrogen Chloride, and Carbon monoxide)