a
(a) 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 move. The body \(A\) will start moving when pseudo force is more than friction force.

i.e. for slipping, \({m_A}a = \mu \;{m_A}g\)

\(\therefore a = \mu \,g\)