The musical instrument referred to as the "king of instruments" is the a. human voice. b. organ. c. tuba. d. violin.

The organ.

Explanation.
There is a lot of controversy about whether the organ or the piano should be considered as the "King of instruments".
Some of the reasons why the pipe organ wins the honor include:
(a). Longevity. The origin of the organ goes as far back as the 3rd century BC.
(b) It has a longer history than any other musical instrument for its usage in churches, synagogues, concert halls, and many public places.
(c) A lot of classical and historical music was written by renowned composers such as Bach and Handel for the organ.

Although the piano competes with the organ in many respects, the organ wins the nod because of longevity.

The appropriate answer is b. the organ. An organ is a large musical wind instrument having rows of pipes. The pipes of the organ are sounded by compressed air. The pipes are controlled or sounded from a large musical keyboard. The organ is known as the king of instruments because of its wide musical range. 

Amusical im pretty sure