Select the most appropriate idiom that can substitute the underlined segment in the given sentence.
Reena decided not to go to work today because she was feeling sick.

The correct answer is "under the weather".

Step-by-step explanation:
An idiom is a phrase or expression whose meaning cannot be understood from the literal definition of its individual words. Let's analyze the meaning of the underlined segment "feeling sick" and compare it with the given options to find the correct idiomatic substitute:

1. "under the weather": This is a common English idiom that means feeling slightly sick, ill, unwell, or tired. In the context of the sentence, Reena decided not to go to work because she was "feeling sick", which maps perfectly to feeling "under the weather".
2. "fit as a fiddle": This idiom means to be in very good health or physical condition, which is the opposite of feeling sick.
3. "sitting on the fence": This idiom means being undecided or avoiding making a decision between two choices. It has nothing to do with health or sickness.
4. "going down in flames": This idiom means to fail spectacularly or completely. It does not relate to physical health or feeling sick.

Therefore, the most appropriate idiom to substitute the underlined segment "feeling sick" is "under the weather".