NEGATIVES WITH COMPARATIVES [closed]

This listening question is confusing me

(woman): Have you gotten over your cold yet?
(man): I couldn’t be feeling any better today.
(narrator): What does the man mean?

a. He’s feeling terrific.
b. He felt a lot worse today.
c. He’s not feeling too well today.
d. He’s a bit better today.

The correct answer is (a) which means that the man feels wonderful today. The sentence "I couldn’t be feeling any better today." gives me the opposite meaning, that he can't feel any improvement today.

Can you explain it?

If this sentence means that he feels wonderful, what sentence implies that he can't feel any improvement today?

The sense is "I feel so well that it would be impossible for me to feel any better."

Compare "I couldn't care less," meaning "I care so little that it would be impossible for me to care any less." (Some Americans say "I could care less", which makes no sense in context.)