16. These structures keep the atrioventricular valves from flipping back into the atria.
a. Chordae tendineae
b. Myocardial septa
c. Purkinje fibers
d. Semilunar cusps

The structures that prevent the atrioventricular valves from reopening into the atria are called chordae tendineae (option a).

Explanation:

Tendinous cords, or chordae tendineae, are structures in the form of fibrous filaments that connect the atrioventricular valves to the papillary muscles of the ventricles.

The function of the chordae tendineae is to prevent the mitral and tricuspid valves from opening during ventricular contraction, preventing backflow of blood from the ventricles to the atria.

The chordae tendineae are predominantly made up of collagen and elastin, in addition to a small percentage of endothelium.

The other options are not correct because:

    b. Myocardial septa is the division between the right and left ventricles.

    c. Purkinje fibers are fibers that transmit the nerve impulse that allows ventricular contraction.

    d. Semilunar cusps, correspond to the shape of the closed semilunar valves.

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