The correct answer is: C. 60° and 10°.
The pitch of a staircase is the ratio of the rise to the run. The rise is the vertical distance between two consecutive treads, and the run is the horizontal distance between the two treads. The maximum pitch of a staircase is 60°, and the minimum pitch is 10°. A staircase with a pitch greater than 60° is considered to be too steep and dangerous, while a staircase with a pitch less than 10° is considered to be too shallow and difficult to use.
Option A is incorrect because 90° is a right angle, and a staircase with a right angle would be impossible to climb.
Option B is incorrect because 75° is greater than the maximum pitch of 60°.
Option D is incorrect because 40° is less than the minimum pitch of 10°.