Explanation:

- The sentence talks about butter that has been in the refrigerator for a long time. It's likely that the butter has gone bad.

- Option 1: Riveting - Usually means something that is very interesting or engaging. Not suitable for describing butter.

- Option 2: Complacent - Refers to self-satisfaction or being unconcerned. Doesn't fit the context of stale butter.

- Option 3: Rancid - Describes something, especially food, that has gone bad or has an unpleasant smell or taste due to being stale. Correct Option: The butter that has been left for years would most likely be rancid.

- Option 4: Dire - Often used to describe a situation that's urgent or dreadful. Doesn't apply to butter directly.