Final answer:
The space between plates in a square butt joint is referred to as the root gap, which is crucial for proper weld penetration and joint strength. Option a is the answer.
Step-by-step explanation:
The space between plates in a square butt joint is called the root gap. This term refers to the distance deliberately left between the edges of the plates to be welded. The root gap is crucial for allowing the welding electrode or filler material to penetrate the joint properly, ensuring a strong weld. In the context of welding, ensuring proper root gap is a key aspect of preparing joints for welding and varies depending on the thickness of the materials and the welding process being used.