A closed cubical box is completely filled with water and is accelerated horizontally towards right with an acceleration a. The resultant normal force by the water on the top of the box.

• A pseudo force ma acts on the water towards the left as a result of the acceleration a towards the right. 
• The normal pressure on the top of the box by the water will vary as illustrated in the coloured figure below. 
• The pressure owing to this force increases from right to left. On top, there is no pressure because there is none due to gravity. 
• At a distance of 1/3rd of the distance from the left side of the triangle, the whole force (N) of this triangular pressure distribution will act.
• N will therefore pass through a point to the top left of the box's centre.

Hence the correct answer is Passes through a point to the left of the centre.