d
Drawing free body - diagrams for \(m \& M\),

we get \(T=m a\) and \(F-T=M a\)

Where \(T\) is force due to spring

\(\Rightarrow F-m a=M a\) or \(, F=M a+m a\)

\(\therefore a=\frac{F}{M+m}\)

Now, force acting on the block of mass

\(m\) is \(m a=m\left(\frac{F}{M+m}\right)=\frac{m F}{m+M}\)