We can get the probability by 1st calculating the area of the shaded triangle region and the area of the square.
Area of triangle = b h / 2
Where,
b = h = half of side length of square = 9 mm
Area of triangle = 9 mm * 9 mm / 2
Area of triangle = 40.5 mm^2
Area of square = s^2
Area of square = (18 mm)^2
Area of square = 324 mm^2
Probability that a point is in the shaded region = Area of triangle / Area of square
= 40.5 / 324
= 0.124
ANSWER: The closest answer is letter C. 0.125