Which of the following statements is incorrect ?

A and c are correct
Only b is incorrect
So answer will be B

Incorrect Statements
a) Error due to refraction may not be completely eliminated by reciprocal levelling - This statement is correct. Refraction is the bending of light as it passes through the atmosphere, which can cause errors in levelling. Reciprocal levelling is a technique used to eliminate the effects of refraction by taking readings in opposite directions and averaging the results. However, this technique may not completely eliminate the effects of refraction.

b) Tilting levels are commonly used for precision work - This statement is incorrect. Tilting levels, also known as dumpy levels, are not commonly used for precision work as they suffer from errors due to the tilting of the instrument. Instead, more precise instruments such as automatic levels or digital levels are used for precision work.

c) The last reading of levelling is always a foresight - This statement is incorrect. The last reading of levelling may be a backsight or a foresight, depending on the method used. In a single levelling, the last reading is a backsight, while in a differential levelling, the last reading is a foresight. d) All of the above statements are incorrect - This statement is incorrect as statement a) is correct. Conclusion The incorrect statement is option 'D', which states that all of the above statements are incorrect. In reality, statement a) is correct, while statements b) and c) are incorrect.

Last reading should be FS.