The question asks which operation most benefits from using a V-block with a clamp. Let's evaluate each option one by one to find the correct answer:

1. Filing flat surfaces: Filing flat surfaces does not require a V-block because a flat stable surface is sufficient for such operations. V-blocks are typically not used for this task.
2. Grinding a rectangular plate: Similar to filing, grinding a rectangular plate does not typically require a V-block. The plate itself provides enough stability on the grinding machine.
3. Drilling a hole through a round bar: This is where a V-block with a clamp shines. The V-block provides stability and holds the round bar securely, preventing it from rolling or moving during the drilling process. This ensures precision and safety during drilling operations.
4. Measuring internal diameter: Measuring internal diameters of components doesn't generally involve V-blocks, as it often requires measuring tools like calipers or bore gauges.

Based on this analysis, option Drilling a hole through a round bar is the correct answer because the V-block provides the necessary support and stability for securing round objects during drilling.