Which of the following is the least black body?

Options

A. Coal
B. Sun
C. Ferry's black body
D. Wien's black body

Explanation

To determine which of the given options is the least black body, we need to understand the concept of black bodies and emissivity.

### Step-by-Step Solution:

1. **Understanding Black Bodies**: A black body is defined as an idealized physical object that absorbs all incoming radiation, regardless of frequency or angle of incidence. It has an emissivity (ε) of 1, meaning it is a perfect emitter of radiation.

2. **Identifying the Options**: The options provided are:
- Coal
- Sun
- Black body (ideal)
- Black body (ideal)

3. **Analyzing the Sun**: The Sun is often considered a perfect black body for practical purposes, as it emits radiation across a wide spectrum and can be approximated with an emissivity of 1.

4. **Identifying Real Bodies**: Coal is a real material and does not perfectly absorb or emit radiation. Its emissivity is less than 1, indicating that it is not a perfect black body.

5. **Comparing Emissivity**: Since the ideal black bodies (the last two options) have an emissivity of 1, and the Sun is also treated as a perfect black body, we need to compare these with coal. Coal has an emissivity that is significantly less than 1.

6. **Conclusion**: Among the options, coal has the least emissivity, making it the least black body compared to the others listed.

### Final Answer:
**Coal is the least black body among the options provided.**