**Answer:**

In the preliminary survey of a road project in a hilly region, cross sections are taken at regular intervals to gather information about the terrain and topography. These cross sections help in determining the vertical alignment of the road and are crucial for designing the road profile.

The correct answer to the question is option 'D', which suggests that cross sections are generally taken at 30 m intervals. This means that a cross section is taken every 30 meters along the road alignment.

**Reasons for taking cross sections every 30 meters:**

1. **Accuracy of Data:** Taking cross sections at closer intervals ensures a more accurate representation of the terrain. This is particularly important in hilly regions where the topography can change rapidly.

2. **Slope Calculation:** Cross sections at 30 m intervals allow for precise calculation of slope gradients. This information is crucial for designing road alignments that are safe and manageable for vehicles.

3. **Identification of Drainage Patterns:** Cross sections help in identifying natural drainage patterns and water flow directions. This information is essential for designing effective drainage systems that prevent water accumulation on the road surface.

4. **Identification of Cut and Fill Areas:** Cross sections provide information about areas that require cut or fill during road construction. By taking cross sections at regular intervals, engineers can accurately estimate the volume of earthwork needed for the project.

5. **Safety Considerations:** In hilly regions, the road alignment needs to be carefully designed to ensure the safety of drivers. Taking cross sections at closer intervals helps in identifying potential hazards such as steep slopes, sharp curves, or unstable terrain.

6. **Cost Optimization:** By taking cross sections at 30 m intervals, engineers can optimize the cost of the project by minimizing unnecessary excavations or fillings. This ensures that road construction is carried out efficiently and within budget.

In conclusion, cross sections are taken at 30 m intervals in the preliminary survey of a road project in a hilly region. This interval allows for accurate data collection, slope calculation, identification of drainage patterns, estimation of earthwork volumes, safety considerations, and cost optimization.

Cross sections taken in hilly terrain is 30m as per IRC Therefore ans should be (b)