What tool is primarily used for marking straight lines in construction?

The chalk line reel is the correct tool for marking straight lines in construction. It consists of a string coated with chalk powder that, when pulled tight and snapped, leaves a straight chalk line on the surface. This is particularly useful for laying out foundation lines, walls, and other structural elements that require precise alignment.

While a level is used to ensure surfaces are horizontal or vertical, it does not assist in marking lines. The measuring tape is invaluable for measuring distances but does not inherently create lines. A square is primarily used for ensuring right angles and checking the squareness of corners, but it also doesn’t serve the function of marking lines over distances. Thus, the chalk line reel stands out as the specific tool designed for the task of marking straight lines.