A piece of ____ clamped to the work may be used to achieve straighter kerf line when cutting manually.

Using angle iron clamped to the workpiece is effective for achieving a straighter kerf line when cutting manually due to its rigidity and structural stability. Angle iron typically features a 90-degree angle, providing a reliable straight edge that helps guide the cutting tool along a specified path. This prevents deviation and allows for more precise cuts, which is particularly beneficial in welding and fabrication tasks where accuracy is critical.

The other materials listed may not provide the same level of support or guidance. Aluminum tubing, while sturdy, is not flat and may not effectively serve as a straight edge. Hardened brass may not be as accessible or practical for this purpose, and PVC, being a plastic, lacks the weight and rigidity needed to maintain a straight cutting line.