Not a method used for preparing approximate estimates:

The correct answer is option 'D' - Cylindrical base method.

Explanation:

There are various methods used for preparing approximate estimates in civil engineering. These methods help in obtaining a rough estimate of the cost of construction before the detailed design and drawings are completed. The approximate estimates are useful for budgeting purposes and determining the feasibility of a project.

Let's discuss the three methods mentioned in the options and explain why the cylindrical base method is not a valid method for preparing approximate estimates:

1. Cubical contents method:
- This method involves estimating the cost based on the volume of the building or structure.
- It considers the length, width, and height of the structure to calculate the approximate cost.
- It is commonly used for estimating the cost of buildings with regular shapes, such as rectangular or square buildings.
- The cost is determined by multiplying the volume of the structure by the rate of construction per unit volume.

2. Unit base method:
- This method involves estimating the cost based on the cost per unit area of the building or structure.
- It considers the total built-up area of the structure, including all floors and rooms.
- The cost is determined by multiplying the total built-up area by the rate of construction per unit area.
- This method is commonly used for estimating the cost of residential and commercial buildings.

3. Plinth area method:
- This method involves estimating the cost based on the plinth area of the building or structure.
- The plinth area is the sum of the built-up area of all floors at the plinth level.
- The cost is determined by multiplying the plinth area by the rate of construction per unit area.
- This method is commonly used for estimating the cost of residential buildings and small commercial structures.

4. Cylindrical base method:
- The cylindrical base method is not a valid method for preparing approximate estimates.
- There is no such method in civil engineering that specifically uses a cylindrical base for estimating costs.
- The cylindrical base method mentioned in option 'D' is not a recognized or commonly used method in the field.

Therefore, the correct answer is option 'D' - Cylindrical base method.