Assertion (A): The planet Neptune appears blue in colour.

Reason (R): The presence of Methane gas in the atmosphere of Neptune is responsible for its colour.

A is false, but R is true.

The correct answer is Option (4) - A is false, but R is true.

The major constituents of planet Neptune are methane, helium and hydrogen gas, and the methane gas plays an important role in providing colour to the planet Neptune. We know that when a white light is passed through the prism it gets converted into seven colours known as VIBGYOR, which represents the colour of rainbow. The same phenomena happens here when the white light is passed through the methane gas molecules, molecules they absorb the red light and reflects the blue light from them which imparts the blue colour to its environment. The scattering of blue light is maximum because in VIBGYOR, the increasing order of wavelength is seen and blue colour has the shorter wavelength which scatters the most. Thus the assertion statement is true and the reason statement is also true which says that the Neptune planet is blue in colour because of the presence of the methane gas in its environment.