A heart murmur is, at its core, a sound problem caused by a flow problem, turbulent blood movement inside or around the heart, rather than by anything electrical.
That distinction rules out two of the options right away. The SA node and AV node are both parts of the heart's electrical conduction system, the SA node sets the pace of each heartbeat and the AV node relays that signal to the ventricles. Trouble in either one shows up as an abnormal rhythm or a conduction delay, not as an audible whooshing sound. The pulmonary vein, meanwhile, is just a low-pressure conduit returning oxygenated blood to the left atrium and is not a typical site where the kind of turbulence that produces a murmur develops.
Heart valves are different because they are mechanical gates that blood must pass through in a specific direction at specific points in the cardiac cycle. If a valve becomes narrowed, blood is squeezed through a smaller opening at higher velocity, and if a valve fails to close fully, blood leaks backward against the normal flow. Both situations create the kind of turbulent, disordered flow that a stethoscope picks up as a murmur.
Since murmurs are a flow phenomenon centred on the valves, heart valves is the correct answer.