Final answer:
The welding method that matches the description of being able to weld thick material in a single pass, having a high investment cost, and being mostly used with low carbon steel is Submerged Arc Welding (SAW).
Step-by-step explanation:
The method described in the student's question that can weld thick material in a single pass, has a high investment cost, and is mostly used with low carbon steel is Submerged Arc Welding (SAW). This type of welding process is known for its ability to handle thick materials efficiently. In the SAW process, the welding arc is submerged under a layer of flux, which helps to protect the weld zone from atmospheric contamination, and the high heat concentration allows deep weld penetration in a single pass. It is also known for its high quality, productivity, and requires less operator skill compared to other methods such as TIG or MIG welding.