What does this expression mean: "Count on me"?

[KYTTIE]

Test No. incompl/elem-29 "Responses (2)", question 4

Mike: 'I need some support, right now.'
Jane: '.........'

(a) Well, you can number me.
(b) Well, I'm your number.
(c) Well, you can count on me.
(d) Well, you can add on me.

Test No. incompl/elem-29 "Responses (2)", answer 4

Mike: 'I need some support, right now.'
Jane: 'Well, you can count on me.'

Correct answer: (c) Well, you can count on me.

Your answer was: incorrect
Mike: 'I need some support, right now.'
Jane: 'Well, I'm your number.'
_________________________

Hi,
Please explain more on this expression?
Thanks a lot
KYTTIE

[yo]

both mean that you can rely on me, or I'm here for you if you need me. Showing moral support.

[snowwhite]

both mean that you can rely on me, or I'm here for you if you need me. Showing moral support. :

But why "Well, I'm your number." in here is not the answer.
Please explain more.

Thanks,

snow