Why are the passengers in the upper deck of the double-decker bus not allowed to stand?
(A) This ensures that the center of gravity of the system may not rise up and the bus may not to be toppled due to unstable equilibrium
(B) This ensures smaller centripetal force, thus helping the driver to negotiate the roundabouts properly
(C) If the passengers are in standing position, they may start oscillating due to jerks and there is a possibility of resonance, causing the bus to be toppled
(D) This is just for the safety reason

Hint: Passengers standing in the upper portion of the double-decker bus increase the altitude of their own center of gravity. The bus faces a lot of vibrations and sudden motions. The linear acceleration is directly proportional to the distance of the center of the mass from the pivot point (which is the ground). This leads to larger jerks which could make a passenger fall.

Complete step by step answer:
Centre of gravity: It is a point where the weight of the body is balanced.
In a double-decker bus, when a person stands in the upper deck of a bus, the center of gravity of a person is also raised. Any angular motion that the bus makes will be experienced by the person as linear motion. Let’s say that there is an angular acceleration \[\alpha \] . The linear acceleration experienced by the person is going to be \[a = \alpha .r\] which is more when the person is standing since the r is more. This means that the passenger can fall and get hurt!

Option D is correct.

Note:
The options are too similar apparently.
The next closest option is B. Smaller Centripetal force makes it easy for the driver to take turns. However, the question asks for the reason why the passenger should not be standing in the upper deck. So, although the option is correct but doesn’t answer the question directly. Hence Option D is the only closest correct option.