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Point A

Explanation:

There are 3 states of equilibrium, which are as follows:

1. Stable Equilibrium

2. Unstable Equilibrium

3. Neutral Equilibrium

1. Stable Equilibrium:

A body is said to be in stable equilibrium, if it comes back to its original position, when it is slightly  displaced. In this case the center of mass of the body is raised up as compared to the initial position.

Example: A book lying on a table, a map hanging on the wall, a stone suspended by a spring.

2. Unstable Equilibrium:

A body is said to be in unstable equilibrium if it does not come back to its original position when it is  slightly displaced. In this case the center of mass of the body is lowered, as compared to the initial position.

Example: A vertical standing pencil, a stick balanced on finger, a funnel resting on its pointed end.

3. Neutral Equilibrium:

A body is said to be in neutral equilibrium, if on being slightly displaced it does not come back to its  original position but occupies a new position similar to its original position. In this case the center of mass of the body remains at same height.

Example: A spherical ball on floor, a cone resting in horizontal position.

So, it is clear from the figure that when we will remove displacement the bus will come back to its original position. Therefore, this is the case of stable equilibrium.

So, the center of mass must have raised up. The original center of mass of the bus must be in the center. So, now the center of mass should have moved above center of bus, vertically.

Therefore, the center of mass of the bus is at point A  

this is poiselle's equation from fluid mechanic in physics. mech eng