In hilly terrain, the final survey requires the driving of centre line stakes at regular intervals. The correct interval between two stakes is given by option 'B', which is 20 m.

Explanation:

Centre line stakes are driven during the final survey to mark the center line of the road or any other structure. In hilly terrain, it is important to place these stakes at regular intervals to ensure that the road or structure is properly aligned and constructed.

The interval between two stakes depends on several factors such as the terrain, the length of the road, and the accuracy required. In general, the interval between two stakes should be small enough to ensure that any errors or deviations from the desired alignment can be detected and corrected.

Option 'B' suggests an interval of 20 m between two stakes. This means that a stake is driven every 20 m along the center line. This interval is suitable for hilly terrain where the slope and elevation changes frequently.

Advantages of driving center line stakes at regular intervals:

1. Ensures proper alignment: The center line stakes help to ensure that the road or structure is properly aligned and constructed.

2. Detects errors: Any errors or deviations from the desired alignment can be detected and corrected by referring to the center line stakes.

3. Accurate measurement: The regular interval between the stakes ensures accurate measurement of the road or structure.

4. Easy identification: The center line stakes make it easy to identify the center line of the road or structure during construction.

Conclusion:

The correct interval between two center line stakes in hilly terrain is 20 m. This ensures proper alignment and accurate measurement of the road or structure. The regular placement of center line stakes also helps to detect and correct any errors or deviations from the desired alignment.