Final answer:
The slope of AC is 1.
Step-by-step explanation:
The slope of AC can be found by using the formula:
slope = (y2 - y1) / (x2 - x1)
Substituting the coordinates of points A(a,0) and C(0,a) into the formula, we get:
slope = (0 - a) / (0 - a) = 1
Therefore, the value that belongs in the green box is: a) 1