Which tool in Illustrator is primarily used to create straight lines?

The Line Segment Tool is specifically designed for creating straight lines in Adobe Illustrator. When you select this tool and click and drag on the canvas, it allows you to create precise straight lines of varying lengths and angles. This tool is particularly useful for drafting and designing, where the need for straight lines is common.

While the Pencil Tool allows for freeform drawing of lines, it does not guarantee straightness and is more suited for creating organic and sketch-like shapes. Similarly, the Brush Tool is used to create strokes that typically follow the shape of the brush selected, allowing for more artistic flexibility rather than straight, geometric lines. The Rectangle Tool, on the other hand, is intended for creating rectangular shapes rather than simple lines, making it unsuitable for the specific task of drawing straight segments. Thus, the Line Segment Tool stands out as the precise tool for this purpose in Illustrator.