Answer:
AC
Explanation:
We know that a 'diagonal' is the line segment that joins the two vertices of a figure, where those vertices lie on the opposite edges.
i.e. any line segment that slopes in the figure is called a diagonal.
Now, as we can see from the given figure that line segment AS, BT and CQ are sloping inside the figure and are joining the opposite vertices.
Hence, these three are diagonals.
Whereas, the line segment AC does not slope and also both the vertices A and C lie on the same edge.
Hence, AC is not a diagonal.