The correct answer is: A. 45° only.
A French cross staff is a navigational instrument used to measure the angle between two objects. It consists of two arms, one fixed and one movable, that are joined at a right angle. The movable arm is marked with degrees, and the fixed arm is marked with a scale of distances. To use the French cross staff, the user aligns the fixed arm with one object and the movable arm with the other object. The angle between the two arms is then read off the scale.
The French cross staff can only be used to measure angles that are multiples of 45 degrees. This is because the movable arm is only marked with degrees, and the fixed arm is only marked with a scale of distances. Therefore, the French cross staff cannot be used to measure angles that are not multiples of 45 degrees.
Option B is incorrect because the French cross staff can be used to measure angles that are multiples of 45 degrees, but not any angle.
Option C is incorrect because the French cross staff can only be used to measure angles that are multiples of 45 degrees, not either 45° or 90°.
Option D is incorrect because the French cross staff can only be used to measure angles that are multiples of 45 degrees, not any angle.