Correct Answer: something that causes friction

Solution : The second option is the correct choice.

The idiom apple of discord refers to a person or thing that causes conflict or discord among others. It originates from Greek mythology, where a golden apple inscribed with "for the fairest" led to a dispute among goddesses, ultimately causing the Trojan War. In modern usage, it symbolises a catalyst for strife or disagreement.

Therefore, the correct meaning is, something that causes friction.