The primary mechanical feature of a turbine is a rotatable element- called the runner, rotor, or wheel- that is mounted on a shaft and has a series of vanes, blades, or buckets arranged around [it's] periphery. When the moving fluid strikes the blades or buckets, the energy of the fluid is transferred to the wheel, causing rotation of the wheel and the turbine shaft.

The word "its" refers to ..... .
1- shaft2- element3- turbine4- feature