Scribing block is used to

Option A: Hold the round bars during marking A scribing block is not meant to hold workpieces. Its design includes a heavy base and a scriber, but no clamping mechanism. For holding round bars, a V-block or other holding fixture is used. Therefore, this option is incorrect.

Option B: Check the trueness of flat surfaces While a scribing block can be moved across a surface plate to detect variations in height, this is not its primary or most accurate function. Dedicated tools like surface plates, straight edges, or dial indicators are used to check the trueness of flat surfaces. Hence, this option is misleading and not correct in the context of its main purpose.

Option C: Locate the centres of round bars This is the correct function. A scribing block, often used along with a V-block, is ideal for locating and marking the centre lines on cylindrical workpieces. By adjusting the scriber and rotating the bar, accurate centre marking can be achieved, which is essential for machining operations.

Option D: Check the surface roughness Surface roughness is determined using roughness comparators or profilometers, not a scribing block. A scribing block has no capability to measure or compare the surface finish quality, so this option is incorrect.

Conclusion: After evaluating all options, it is clear that the primary and correct use of a scribing block is to locate the centres of round bars. Therefore, the correct answer is Option C.