The answer is option 'B' - Boom.

Explanation:

Compression Member:
A compression member is a structural element that is subjected to a compressive force, causing it to decrease in length. It is designed to withstand and transfer the compressive loads without buckling or failure. Compression members are commonly used in various structures, such as buildings, bridges, and trusses.

Types of Compression Members:
There are several types of compression members used in structural engineering. Some of the common examples include:

1. Columns: Columns are vertical compression members that support the weight of a structure and transfer loads to the foundation. They are typically used in buildings and other vertical structures.

2. Struts: Struts are compression members that are used to support or brace components of a structure. They are often used in trusses, frames, and bracing systems.

3. Purlins: Purlins are horizontal structural members that support the roof covering and transfer the roof loads to the main structural members. They are typically used in roof construction.

4. Girts: Girts are horizontal structural members that provide support for the exterior walls of a building. They are commonly used in metal building systems.

5. Boom: A boom is a compression member used in cranes or other lifting devices. It is responsible for supporting the load and transferring it to the crane's base or other supporting structures.

Selection of Boom as the Answer:
Given the options, the only one that represents a compression member is the boom. Purlin, girt, and tie are not compression members but rather different types of structural elements used for different purposes.

- Purlin: Purlins are horizontal members that support the roof covering and transfer the roof loads. They are subjected to bending and are not primarily designed to resist compressive forces.

- Girt: Girts are horizontal members that provide support for the exterior walls. They are subjected to bending and are not primarily designed to resist compressive forces.

- Tie: A tie is a structural member used to resist tensile forces and prevent separation or spreading of structural components. It is not a compression member.

On the other hand, a boom is a compression member used in cranes or lifting devices. It is designed to withstand compressive forces and transfer them to the crane's base or other supporting structures.