Explanation:

The V-block is a precision tool used in various industries, including engineering and manufacturing, to check the trueness of a flat surface. It consists of a pair of V-shaped grooves that are positioned at a right angle to each other, forming a V-shape when viewed from the top.

Trueness of Flat Surface:
The main purpose of a V-block is to ensure that a flat surface is true, meaning that it is perfectly aligned and even. When a flat surface is placed on the V-block, the V-shaped grooves provide support and stability. By using a height gauge or a dial gauge, the user can measure any variations or irregularities in the surface. If the surface is true, the measurements will be uniform and consistent. On the other hand, if there are any deviations or unevenness, the measurements will vary, indicating that the surface is not flat or true.

Other Uses of V-Block:
While the primary use of a V-block is to check the trueness of a flat surface, it can also be used for other purposes, such as:

- Locating Centers of Round Bars: By placing a round bar in the V-block, the user can determine the center of the bar. This is useful when drilling or machining the round bar to ensure precise and accurate positioning.

- Holding Workpieces: V-blocks are often used in conjunction with other tools, such as clamps or vises, to securely hold and position workpieces during machining or inspection processes.

- Checking Roughness: Although not its primary purpose, a V-block can also be used to check the roughness of a surface. By running a finger or a piece of cloth along the surface, the user can feel any irregularities or rough spots. However, it is important to note that specialized tools, such as surface roughness testers, are more accurate and precise for this purpose.

Therefore, the correct answer is option 'B' - To check the trueness of a flat surface.