Correct Answer: Bone of contention

Solution : The third option is the correct answer.

The idiom "apple of discord" refers to a source of conflict or discord, something that provokes disagreement or strife among people. This is synonymous with the meaning of "bone of contention", which also signifies a subject or issue that leads to disagreement or dispute. In both cases, the focus is on something that causes contention or conflict among individuals.

Therefore, the correct answer is bone of contention.