Blue colour of the sky is due to
a. Adsorption of light by dust particles
b. Reflection of light by dust particles
c. Scattering of light by dust particles
d. Pressure of clouds which are a colloidal dispersion of water particles in air

Explanation

To determine why the sky appears blue, we can analyze the options provided in the question step by step.

### Step 1: Understanding the Phenomena
The blue color of the sky is primarily due to the interaction of sunlight with the molecules and particles in the Earth's atmosphere. This interaction can involve various phenomena such as adsorption, reflection, and scattering.

### Step 2: Analyzing Each Option
- **Option a: Adsorption of light by dust particles**
- Adsorption refers to the process where atoms, ions, or molecules from a gas, liquid, or dissolved solid adhere to a surface. This process does not explain the blue color of the sky, as it does not involve the scattering of light.
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