Question

what is parallax?
the shift towards the red end of the electromagnetic spectrum.
changes in the apparent brightness of a star.
a binary star system.
the apparent shift in position caused by the motion of an observer

Explanation:

Parallax is defined as the apparent shift in the position of an object when viewed from different positions (due to the motion of the observer). The first option describes redshift. The second option is related to apparent magnitude changes. The third option is about a system of two stars.

Answer:

The apparent shift in position caused by the motion of an observer.