Which constraint would be used to align two linear elements?

The collinear constraint is specifically designed to align two linear elements, which can include lines or edges. By applying this constraint, you ensure that the selected elements remain in a single straight line, meaning they share the same line of action or direction. This is particularly useful in geometric design with Solid Edge, as it maintains the intended alignment of parts or components within an assembly with precision.

Other constraints mentioned, such as perpendicular and equal, serve different purposes. A perpendicular constraint would create a 90-degree angle between two elements, while an equal constraint would force two elements to have the same length. The 2D constraint is too broad and does not specifically address the alignment of linear elements. Thus, using the collinear constraint directly achieves the goal of alignment in a straightforward and effective manner.