Which of the following is not a valid method to create a straight line in a sketch?

The method of using a compass is not valid for directly creating a straight line in a sketch. A compass is primarily designed to draw circles or arcs and does not have a mechanism for producing straight lines.

In contrast, a ruler is explicitly used for measuring and drawing straight lines due to its flat edge, which aids in ensuring accuracy and precision. A straightedge, similar to a ruler but without measuring marks, can also create straight lines effectively. A drawing board, while not a tool for drawing lines per se, provides a stable surface and supports the use of tools like rulers or straightedges, facilitating the drawing process.

Thus, the only option that does not directly facilitate the creation of a straight line is the use of a compass.