The correct answer is option 'B' - The back staff is at a lower point.

Explanation:
When we are using a levelling instrument to determine the difference in height between two points, we take readings on a staff held vertically at each point. The staff readings taken in the direction of the line of sight are called foresights, and the staff readings taken in the opposite direction are called backsights.

In this question, it is given that the back sight is more than the foresight. This means that the reading taken on the staff at the point where the instrument is set up (back sight) is greater than the reading taken on the staff at the point where the instrument is moved to (foresight).

To understand why the correct answer is option 'B' - The back staff is at a lower point, let's consider a simple example:

Suppose we have two points A and B, and the instrument is set up at point A. The staff reading at point A is taken as the back sight, and the staff reading at point B is taken as the foresight.

If the back sight is greater than the foresight, it means that the instrument is set up at a higher point (A) compared to the point where the foresight is taken (B). This is because the staff reading decreases as we move up the staff. So, if the back sight reading is greater, it indicates that the instrument is at a higher elevation than the foresight point.

Therefore, option 'B' - The back staff is at a lower point is the correct answer.

It is important to note that the difference in level between the two points can be calculated by taking the algebraic difference between the back sight and the foresight. This difference can be positive if the back sight is greater or negative if the foresight is greater. However, this information alone does not tell us which point is at a higher elevation.