Effort is a ……….

Options

A. exerted on machine
B. supplied by wind
C. external agency supplied to the machine.
D. created by the machine.

Explanation

To answer the question "Effort is a ……….", we need to analyze the options provided and understand the concept of effort in the context of machines.

### Step-by-Step Solution:

1. **Understanding the Concept of Effort**:
- Effort refers to the force applied to a machine to perform work. In simple machines, this effort is necessary to overcome the load.

2. **Identifying the Load**:
- In the example of a pulley lifting a bucket, the load is the weight of the bucket filled with water. This weight is the force that the machine needs to overcome.

3. **Defining the Source of Effort**:
- The force that a person applies to the machine (in this case, pulling the string of the pulley) is what we refer to as effort.

4. **Evaluating the Options**:
- **Option A**: Exerted on machine - This is incorrect because effort is not something that is exerted on the machine; it is applied by a person.
- **Option B**: Supplied by wind - This is also incorrect as effort is not supplied by wind; it is a human action.
- **Option C**: External agency supplied to the machine - This is correct because effort is indeed the force applied by an external source (like a person) to the machine.
- **Option D**: Created by the machine - This is incorrect as the machine does not create effort; it utilizes the effort applied to it.

5. **Conclusion**:
- Based on the analysis, the correct answer is that effort is an external agency supplied to the machine.

### Final Answer:
Effort is an external agency supplied to the machine.