Heart valve leaflets are mainly composed of:

Heart valve leaflets are primarily composed of dense connective tissue and endocardium. The structure of heart valve leaflets plays a crucial role in their function, which requires them to be strong and flexible enough to withstand the forces exerted during the cardiac cycle while also allowing for proper blood flow.

Dense connective tissue provides the necessary strength and stability to the valve leaflets, enabling them to resist the high pressures during ventricular contraction and act effectively as barriers to backflow. This tissue type is rich in collagen fibers, which contribute to the mechanical integrity of the valves.

The endocardium, which is the inner lining of the heart chambers, also contributes to the valve structure. It covers the dense connective tissue and provides a smooth surface that decreases turbulence as blood flows through the heart, promoting efficient circulation.

Given the essential roles of both dense connective tissue and endocardium in maintaining the structural integrity and function of heart valves, this combination is what makes option B the correct answer.