The correct answer is: C. Levelling staff
A levelling staff is a long, straight rod with a series of evenly spaced marks on it. It is used to measure the difference in elevation between two points. To measure the depth of excavation, the levelling staff is placed on the ground at the bottom of the excavation and then a second levelling staff is placed on the ground at the top of the excavation. The difference in the elevation of the two staffs is then measured using the marks on the staffs.
A ranging rod is a shorter, straight rod that is used to mark out the position of foundations. It is not used to measure the depth of excavation.
A steel tape is a flexible tape that is used to measure distances. It is not used to measure the depth of excavation.
A boning rod is a short, straight rod that is used to align walls and other structures. It is not used to measure the depth of excavation.