Final answer:
In welding, the triangle symbol (option a. △) indicates that a plate is to be beveled prior to welding, which is essential for the quality of the weld.
Step-by-step explanation:
The welding symbol that specifies the plate that is to be beveled is a triangle (option a. △). This symbol is part of the standardized set of symbols used in welding blueprints and diagrams to indicate the type of weld that is to be performed. When a triangle is present, it signifies that the metal in question should be prepared with a beveled edge before welding. This preparation allows for better penetration of the weld material into the base metals and is critical for the strength and integrity of the weld.