Correct option is (4) closure of the tricuspid and bicuspid valves
The correct answer is option (4) because LUB is the first heart sound and it is produced by closing of AV valves; (tricuspid valve in between right atrium and right ventricle) and (bicuspid /mitral valve in between left atrium and left ventricle), during ventricular systole.
Option (1) is incorrect as closure of the semilunar valves produce the second heart sound called ‘DUB’.
Option (2) is incorrect because opening of valves are not responsible for production of heart sounds ‘LUB’ and ‘DUB’.
Option (3) is incorrect because opening of semilunar valves does not produce only heart sounds.
