### 1. Identify the Problem
The question asks for the SI unit of efficiency from the provided options.

### 2. Relevant Concepts
Efficiency is a measure of how well input energy is converted into output energy. It is calculated as:

$$
\text{Efficiency} = \frac{\text{Useful Output Energy}}{\text{Input Energy}} \times 100\%
$$

Since efficiency is a ratio of two energies, it is dimensionless, meaning it does not have any units.

### 3. Analysis and Detail
- **Newtons**: This is the SI unit of force.
- **Dimensionless**: This means it has no units; it can be a ratio or a proportion.
- **Kelvins**: This is the SI unit for temperature.
- **Joule**: This is the SI unit of energy.

Since efficiency is a ratio of energies, it is dimensionless.

### 4. Verify and Summarize
Reviewing the definition, we confirm that efficiency compares input and output energies, making it a dimensionless quantity.

### Final Answer
The SI unit for efficiency is **dimensionless**.

Question

### 1. Identify the Problem
The question asks for the SI unit of efficiency from the provided options.

### 2. Relevant Concepts
Efficiency is a measure of how well input energy is converted into output energy. It is calculated as:

$$
\text{Efficiency} = \frac{\text{Useful Output Energy}}{\text{Input Energy}} \times 100\%
$$

Since efficiency is a ratio of two energies, it is dimensionless, meaning it does not have any units.

### 3. Analysis and Detail
- **Newtons**: This is the SI unit of force.
- **Dimensionless**: This means it has no units; it can be a ratio or a proportion.
- **Kelvins**: This is the SI unit for temperature.
- **Joule**: This is the SI unit of energy.

Since efficiency is a ratio of energies, it is dimensionless.

### 4. Verify and Summarize
Reviewing the definition, we confirm that efficiency compares input and output energies, making it a dimensionless quantity.

### Final Answer
The SI unit for efficiency is **dimensionless**.

Knowee AI · Accepted Answer