c
So block $Q$ is moving due to force and block $P$ due to friction.
Friction direction on both blocks is shown in the figure.
The first block $"Q"$ will move and $"P"$ will move with $Q$ so by taking p and
$\mathrm{q}$ as a system.
$F-9=0 \Rightarrow F=9 N$
When $9 \mathrm{N}$ force is applied then acceleration $\mathrm{ap}=\mathrm{aq}=0$
$4Kg$ block is koving due to friction and maximanum friction force is $8 \mathrm{N}$.
So acceleration $=8 / 4=2 \mathrm{m} / \mathrm{s}^ 2$
Block Q has positive acceleeration equal to maximum acceleration of block P i.e. $2 \mathrm{m} / \mathrm{s}^ 2$