Anil could not go to his friendʼs party, as he had fever and was __________.

The sentence describes Anil's inability to attend his friend's party due to illness, specifically mentioning he had a fever. The blank requires an idiomatic expression meaning to feel unwell or be in poor health.

"Under the weather" (Option B) is the correct choice, as this widely recognised idiom precisely means to feel sick or slightly ill. It fits perfectly with the context of Anil missing the party because of his fever. The other options are incorrect because they either misuse the idiom or are nonsensical: "Beneath the weather" (Option A) and "Beside the weather" (Option C) are simply incorrect variations of the established phrase, while "Outside the weather" (Option D) is meaningless in this context and not an English idiom.

Therefore, Option B is the only appropriate answer that preserves both the intended meaning and grammatical correctness of the sentence.