Which of the following is the largest unit of length ?

Options

A. Light year
B. Astronomical unit
C. Parsec
D. Nothing can be decided

Answer

To determine which of the following is the largest unit of length, we will analyze the given options: light year, astronomical unit, parsec, and the option "nothing can be decided".

### Step-by-Step Solution:

1. **Identify the Units of Length**:
- The options provided are:
1. Light year
2. Astronomical unit
3. Parsec
4. Nothing can be decided

2. **Understand Each Unit**:
- **Light Year**: The distance that light travels in one year. It is approximately equal to \(9.46 \times 10^{12}\) meters.
- **Astronomical Unit (AU)**: The average distance from the Earth to the Sun, approximately \(1.5 \times 10^{11}\) meters.
- **Parsec**: A unit of distance used in astronomy, equal to about \(3.09 \times 10^{16}\) meters.

3. **Compare the Units**:
- To compare these units, we can convert them into a common unit (meters) or compare their approximate values:
- 1 Light Year ≈ \(9.46 \times 10^{12}\) meters
- 1 Astronomical Unit ≈ \(1.5 \times 10^{11}\) meters
- 1 Parsec ≈ \(3.09 \times 10^{16}\) meters

4. **Determine the Largest Unit**:
- From the values:
- Light Year: \(9.46 \times 10^{12}\) meters
- Astronomical Unit: \(1.5 \times 10^{11}\) meters
- Parsec: \(3.09 \times 10^{16}\) meters
- Clearly, \(3.09 \times 10^{16}\) meters (1 Parsec) is greater than both \(9.46 \times 10^{12}\) meters (Light Year) and \(1.5 \times 10^{11}\) meters (Astronomical Unit).

5. **Conclusion**:
- Therefore, the largest unit of length among the options is the **Parsec**.

### Final Answer:
The largest unit of length is **Parsec**.