Hello.
When dealing with two or more elements in a fixed proportion, your answer is going to be "B.) Compound." The explanation is pretty basic when you narrow it down - think of it like the English version of a compound sentence; there are two independent clauses that have related ideas (such as this one, for example).
Let's use an example of a compound element, such as
, or Water.
This is a compound element that is composed of the two elements "Hydrogen" and "Oxygen" - for every 2 Hydrogen atoms there is one Oxygen atom.
I hope this helps!