I couldn’t agree with you more! (What's the meaning of this sentence)

[Learnermoh]

Hi Torsten and everyone,

Many thanks for the lessons and for your continuous cooperation.
Could you please explain the following question?

I read this conversation in a book for TOEFL test:
- (woman) That lecture was absolutely fascinating!

- (Man) I couldn’t agree with you more!

- (Question) What does the man mean?

The answer choices:
(a) He prefers not to argue about it.
(b) He disagrees with the woman.
(c) He shares the woman’s opinion.
(d) He wasn’t able to hear the lecture.

The right answer is choice (c).
My question is how choice (c) is correct and not choice (b)? Because the sentence “I couldn’t agree with you more!” is negative as I understood.

Please help me solving this confusion with many thanks.

M.

[Alan]

Hi,

When you say: 'I couldn't agree with you more', you reaaly mean: 'I agree with you completely and it wouldn't be possible for me to agree with you any more.' The negative is usd here to emphasise the agreement.

Alan
_________________
English as a Second Language
You can read my ESL story Present Simple

[Learnermoh]

Hi, Mr Alan,

Thank you very much for your reply, do you mean that there are two negatives in the that sentence so it becomes positive? but if I say "I couldn't agree with you" without the word "more" is the meaning still the same "I agree with".
And are there any differences between these sentences "I couldn't agree with you" and "I didn't agree with you", would you kindly give me more explanation about these questions?

Thanks alot for time and cooperation
Best regards
Moh.