The correct answer is option 'B': sulphuric acid.

Explanation:

The atmosphere of Venus is composed mainly of carbon dioxide (96%) and nitrogen (3.5%), with trace amounts of other gases such as sulfur dioxide, water vapor, and noble gases. However, the most prominent feature of Venus' atmosphere is its thick cloud cover.

1. Composition of Venus' clouds:
- The clouds in Venus' atmosphere are primarily made up of droplets of sulfuric acid. These clouds are thick and extend from about 50 to 70 kilometers above the surface.
- The concentration of sulfuric acid in these clouds is around 75-96%.
- The clouds consist of a mixture of sulfuric acid droplets and other unknown aerosols.

2. Formation of sulfuric acid clouds:
- The sulfuric acid clouds on Venus are formed through a series of chemical reactions in the atmosphere.
- Sunlight provides the energy needed for these reactions to occur.
- The process starts with the dissociation of carbon dioxide and water vapor molecules due to ultraviolet radiation from the Sun.
- The dissociated molecules then react with sulfur dioxide, which is present in small quantities in the atmosphere, to form sulfuric acid.
- The sulfuric acid molecules then combine with water vapor to form droplets, resulting in the formation of the thick clouds.

3. Appearance and properties of the clouds:
- The thick sulfuric acid clouds give Venus its characteristic white and yellowish appearance.
- These clouds reflect a significant amount of sunlight, contributing to the high albedo (reflectivity) of Venus.
- The clouds also create a greenhouse effect, trapping heat and contributing to the extreme temperatures on the planet's surface.

In conclusion, the atmosphere of Venus is primarily composed of thick clouds made up of sulfuric acid droplets. These clouds play a significant role in the climate and appearance of the planet.