Which of the following object will exert the most pressure?

Pressure is given by the perpendicular force exerted per unit area.
$\mathrm{Pressure}=\frac{\mathrm{Force}}{\mathrm{Area}}$

Here, we can see that the pressure is inversely proportional to the area. That means the smaller the area, the higher the pressure. 
In the given options, we can see that a sharp knife is the sharpest object and has a very small area of contact with the object. Therefore, it will exert the highest pressure.