A body B lies on a smooth horizontal table and another body A is placed on B. The coefficient of friction between A and B is $\mu$ .What acceleration given to B will cause slipping to occur between A and B

There is no friction between the body B and surface of the table.

If the body B is pulled with force F, then

$F=(m_A+m_B)a$

Due to this force upper body A will feel the pseudo force in a backward direction.
                                               
$f=m_A\times a$       

But due to friction between A and B , body will not slip . The body A will start slipping when pseudo force is more than friction force. 

i.e for slipping,  $m_{A}a=\mu m_{A}g$

$\therefore a=\mu g$