Q:

I'm waiting for you ON/AT/IN the second floor? 

what is the correct option when I want to sat that I'm waiting for someone ON / IN / AT second floor?  

a) I'm waiting for you on the second floor. 
b) I'm waiting for you in the second floor. 
c) I'm waiting for you at the second floor. 

A:

I'm waiting for you (at the lift, in the hallway) on the second floor. 
Note the article "the" before second floor.