Which valve is present at the opening of coronary sinus?

• Mitral valve

• Eustachian valve

• Thebesian valve

• Tricaspid valve

C.

Thebesian valve

The coronary sinus is a collection of veins joined together to form a large vessel that collects blood from the heart muscle. It delivers deoxygenated blood to the right atrium, as do the superior and inferior vena cava. It returns the blood from the substance of the heart, and is protected by a semicircular fold of the lining membrane of the right atrium, at the orifice of the coronary sinus known as thebesian valve (i.e., valve of coronary sinus). Its main function is to prevent the regurgitation of blood into the sinus during the contraction of the atrium.

The mitral valve is a valve that lets blood flow from one chamber of the heart, the left atrium, to another called the left ventricle. 

The Eustachian valve (also known as the 'valve of the inferior vena cava') is a ridge of variable thickness in the inferior right atrium. It is a remnant of a fetal structure that directed incoming oxygenated blood to the foramen ovale and away from the right atrium.

The tricuspid valve, or right atrioventricular valve, is on the right dorsal side of the mammalian heart, between the right atrium and the right ventricle. Its function is to prevent back flow of blood from the right ventricle into the right atrium.