**A) Neptune**

Neptune is the planet that appears greenish in color. This unique coloration is due to the presence of methane gas in its atmosphere, which absorbs red light and reflects green light. Let's explore this answer in detail:

**Neptune's Atmosphere:**

Neptune is the eighth and farthest known planet from the Sun in our solar system. Its atmosphere is primarily composed of hydrogen and helium, with traces of methane and other compounds. Methane is responsible for Neptune's distinctive blue color.

**Methane and Light Absorption:**

Methane molecules in Neptune's atmosphere have the ability to absorb red light while reflecting and scattering blue and green light. This phenomenon is known as selective light absorption. As a result, the sunlight that reaches Neptune is absorbed by the methane, causing the reflected light to appear greenish-blue.

**Rayleigh Scattering:**

Another factor contributing to the greenish appearance of Neptune is Rayleigh scattering. This scattering phenomenon occurs when the particles in a planet's atmosphere are smaller compared to the wavelength of light. The shorter wavelengths, such as blue and green, scatter more than the longer wavelengths, such as red and orange. This scattering effect enhances the blue and green colors in Neptune's atmosphere.

**Observation and Exploration:**

The greenish appearance of Neptune was first observed and confirmed during the Voyager 2 mission in 1989. The spacecraft provided detailed images and data about Neptune's atmosphere, which helped scientists understand its unique coloration.

**Conclusion:**

In conclusion, Neptune appears greenish in color due to the presence of methane gas in its atmosphere. Methane absorbs red light and reflects green light, causing the reflected sunlight to appear greenish-blue. This distinct coloration, along with the scattering of shorter wavelengths by particles in Neptune's atmosphere, gives the planet its characteristic greenish appearance.