A book is lying on a table. What is the angle between the normal reaction acting on the book on the table…

The normal reaction on the book due to table is contact force which will be perpendicular to contact surface of the book and table, therefore it can be seen in diagram as $N.$

The weight of the book will always act in downward direction, weight is the gravitational force of attraction between the book and earth therefore, its direction is along the centre of earth more specifically its direction is in downward direction perpendicular to table.

Therefore, it is clear that both forces are in opposite direction thus, the angle between them is $\left(90°+90°\right)=180°$