Statement I : The planet Neptune appears blue in colour. Statement II : The presence of methane gas in the atmosphere of Neptune is responsible for its colour.

Neptune is characterized by its distinct deep blue appearance [1]. Its atmosphere is primarily composed of hydrogen and helium, but it also contains a significant amount of methane gas [2]. The blue coloration is a direct result of methane's optical properties; methane molecules in the upper atmosphere selectively absorb red and infrared wavelengths from the incoming sunlight. While the red light is absorbed, the blue light is reflected back into space, giving the planet its characteristic bluish cast. Although Uranus has a similar composition, Neptune's blue is more vivid, likely due to atmospheric dynamics or a thinner haze layer. Therefore, Statement I is true, and Statement II correctly explains the physical mechanism behind the observation, making it the correct explanation.

Sources

1. [1] https://spaceplace.nasa.gov/all-about-neptune/
2. [2] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 2: The Solar System > Neptune > p. 32