Three vessels shown in the figure have same base area. These vessels are filled with same quantity of same liquid. In which vessel will the force on the base be maximum? 

The vessel (iii) is narrowest and vessel (i) is widest at the top. Therefore when the vessels are filled by same quantity of liquid, the height column of liquid in vessel (iii) will be maximum and in vessel (i) is minimum. Thus the pressure due to height column of liquid at the base is maximum in vessel (iii) and minimum in vessel (i).
As the base area of all the vessels is same, hence the force at the base of vessel (iii) will be maximum and that at the base of vessel (i) will be minimum.