A hockey player is moving northward and suddenly turns westward with the same speed to avoid an opponent. The force that acts on the player is

In figure, $\overrightarrow{OA}={\overrightarrow{p}}_1$= initial momentum of player northward.

$\overrightarrow{OB}={\overrightarrow{p}}_2$=final momentum of player westward.

According to triangle law of vectors, $\overrightarrow{OA}+\overrightarrow{AB}=\overrightarrow{OB}$

$\overrightarrow{AB}=\overrightarrow{OB}-\overrightarrow{OA}={\overrightarrow{p}}_2-{\overrightarrow{p}}_1$= change in momentum

The change in momentum of player is along south west.

As motion is due to frictional force of reaction of the ground, therefore, force that acts on the player is frictional force along south west.