Final answer:
The T-square is the appropriate tool for squaring a line across a board, as it allows for drawing straight lines at a 90-degree angle to the edges of the board.
Step-by-step explanation:
Among the tools listed, the T-square is the best option for squaring a line across a board. A ruler can be used for measuring distances or lengths, and while it is important for precision, it doesn't ensure a perfect 90-degree angle. A protractor is used for measuring angles, and a compass is typically used for drawing circles or arcs. The T-square, on the other hand, is specifically designed to assist in drawing straight lines at right angles to the edge of the board, which is why it is the most suitable tool for the task at hand.