(A) : Pressure of the gas increases with the decrease in volume at constant temperature
(R) : Number of molecules per unit volume increases with decrease in volume consequently number of collisions on the wall increases

We know that,

 $$\begin{gathered} \mathrm{PV}=\mathrm{nRT} \\ \mathrm{At} \mathrm{constant} \mathrm{T}, \mathrm{PV}=\mathrm{constant} \\ \Rightarrow \mathrm{P} \mathrm{\alpha } \frac{1}{\mathrm{V}} \end{gathered}$$

Hence, as pressure increases, volume decreases at constant temperature.

The pressure on the walls is due to the collisions of gas molecules on the it. 

As the space occupied by these molecules.i.e., their volume decreases, more number of molecules compete for the space, resulting in increase in collisions on the wall.