ANS:C - the objective and the eye-piece

The effect whereby the position or direction of an object appears to differ when viewed from different positions. Eye piece is only for clearing the vission it is not linked with parallax.
Objective lens adjustment make the object image in the plane of diaphragm. The parallax may be eliminated by properly focussing the telescope so by focusing eyepiece and objective. So given answer is correct.

Ref: As per BC Punmia. The correct answer is only by focussing on the eye-piece. You can remove the parallax error.