Correct option is B
Types of levelling :-
Reciprocal levelling:- When it is not possible to locate the levelling instrument in between the inter visible points, reciprocal levelling is performed. This case appears in case of ponds or rivers etc. In case of reciprocal levelling, instrument is set nearer to 1st station and sighted towards 2nd station.
Fly levelling:- It is conducted when the benchmark is very far from the work station. In such case, a
temporary bench mark is located at the work station which is located based on the original benchmark. Even it is not highly precise it is used for determining approximate level. In fly levelling only backsight and foresight readings are taken at every step of the level.
Differential levelling:- It is the method of levelling to determine the elevation of points located at some distance apart or to determine the elevation difference between two points or to establish
benchmarks. The method is used in order to find the difference in elevations between two points:
