Select the most appropriate meaning of the given idiom.
A bolt from the blue

The correct option is A sudden and unexpected event.

Explanation:

The idiom "A bolt from the blue" refers to a sudden, unexpected, and usually surprising or distressing event or news, coming without any warning, much like a sudden bolt of lightning strikes from a clear blue sky.

Let us look at the meanings of the given options:

1. Raining heavily: Refers to continuous heavy rainfall (often described by idioms like "raining cats and dogs").
2. Thunder and rain in a storm: Refers literally to stormy weather conditions.
3. A sudden and unexpected event: Correctly describes something happening completely without warning.
4. A spontaneous but expected event: Contradicts the unexpected nature of the idiom.

Thus, the most appropriate meaning of the idiom is A sudden and unexpected event.