Oersted discovered a connection between electricity and magnetism when he observed:
A. the motion of a compass needle.
B. a light flickering.
C. a current-carrying wire heating up.
D. a current-carrying wire vibrating.

"the motion of a compass needle." (option A)

Explanation:

Oersted found that when conducting an electrical current through a wire, it was creating a magnetic field that affected a compass near by, and made the compass; needle to change position.

Therefore the correct answer is option A:

"the motion of a compass needle."