Answer:
Lining of the heart ventricle.
Explanation:
Human heart is divided into four chambers: two atrium and two ventricles. The ventricle is located at the bottom of the heart.
The given diagram correspond to the lining of the heart ventricle. The pulmonary circulation of the heart to the lungs occur through the right ventricle. The wall of the ventricles are thicker than atria.
Thus, the correct answer is option (3).