Question

How to align c div on bottom of a?

Position:relative doesnt help.

DEMO

Answer 1

You need to make #c absolutely positioned, and #a relatively positioned:

#a {
    width:250px; 
    height:250px; 
    background:grey;        
    position: relative;
}
#b { }
#c {
    bottom:0px; 
    position:absolute;
}

Updated Example