Concept:Surveying instruments are each designed for a specific task; measuring the angle of a ground slope requires a clinometer.
Explanation:The Watkin's clinometer is a compact instrument with a graduated arc and viewing lens.
It is specially made for measuring vertical angles, such as the angle of a ground slope, in the field.
The optical square is used to set out right angles in chain surveying, not to measure slope angles.
The offset staff is used to measure short distances from a survey line to details, not angles.
The Gunter's chain is used for measuring horizontal distances only.
Therefore, among the given options, the Watkin's clinometer is the correct instrument for slope angle measurement.
Answer:The correct answer is B. The Watkin's clinometer.