Small muscle masses attached to the chordae tendineae are the

Basically, the Atrioventricular valves are Mitral and Tricuspid valves located between the atrium and ventricles. Their main functions is to “prevent the backflow of blood from the ventricles into the atrium during the ventricular contraction (systole).

Explanation:

So all we know is that the student stated something ("if plants are added to an indoor environment the air quality will improve "). This is only a statement - it's not a test whether this statement is true yet.
 
So the correct answer is: it is a hypothesis.