Choose the grammatically correct sentence from the options given:

The correct answer is Option (3) → Yesterday he was down with fever but he is feeling better today.

Tense that matches the time word:

• Clue word "yesterday": a finished past time takes the simple past: "he was down with fever" (he was ill with fever).
• Clue word "today": a situation going on now takes the present continuous: "he is feeling better".
• This sentence: each clause uses the tense that suits its time word, and "but" joins the two contrasting facts.
  Example: Last week the shop was closed, but today it is doing good business.

Why the Other Options are Incorrect?

Option 1: She was more delighted to get the happy news. — Incorrect. "More" forms a comparative, which needs something to compare with ("more ... than"); here nothing is compared.
Corrected: She was delighted to get the happy news.

Option 2: I eat seldom after sunset. — Incorrect. "Seldom" is an adverb of frequency and comes before the main verb.
Corrected: I seldom eat after sunset.

Option 4: The news is much interesting. — Incorrect. "Much" is used with comparatives (much better) and past participles, not with an adjective in the positive degree such as "interesting"; "very" is needed.
Corrected: The news is very interesting.