at or in?
I'm available in afternoon? or at afternoon?
And what is the rule about it?

Question

Answer

(a) never at.
(b) it would be 'in the afternoon'.
(c) If you use the plural, it changes the meaning from an implied 'this afternoon', to all/most afternoons.
(d) You would usually say 'most afternoons'.

Answer

If you are talking about a specific time you can say 'at'
eg. at 12:00 / at dawn/ at midday

If it is a broader period you use 'in the'
eg. in the morning, in the summer, in the afternoon.

Answer

I believe that many people do not use a preposition:

"I am very happy to help you. I am available afternoonS." (The plural of "afternoon" is necessary.)

If you wish to use a preposition, then you might say:

"I am available in the afternoon/ afternoons." (The plural is optional with a preposition.)