In highway and geotechnical engineering, the stability of natural hill slopes depends upon multiple interacting factors. Which of the following groups best captures the main influences on slope stability?

Introduction / Context:
Slope stability is a fundamental topic in highway alignment and geotechnical engineering. Failures such as landslides can be triggered by geometry, geology, and hydrology. A designer must understand how these factors combine before cutting, filling, or situating roads in hilly terrain.

Given Data / Assumptions:

• The question reviews qualitative drivers of slope stability.
• No numerical data are required; principles apply to natural and cut slopes.
• We consider typical highway earthworks in soil or rock masses.

Concept / Approach:
Slope stability depends on resisting forces (shear strength) versus driving forces (self-weight, water pressure, surcharge). Material type and structure (nature of slope), inclination (angle), geologic defects (joints, bedding, faults), and groundwater regime (pore pressure) all alter the factor of safety.

Step-by-Step Solution:

Verification / Alternative check:
Engineers quantify stability via limit-equilibrium or numerical methods. Changing any listed factor (flattening a slope, draining groundwater, or buttressing weak layers) measurably increases the factor of safety, confirming their combined influence.

Why Other Options Are Wrong:
Each single-factor option omits other equally critical influences; slope failures are multi-factor phenomena.

Common Pitfalls:
Ignoring seasonal groundwater rise; assuming rock slopes are always stable; overlooking weak interlayers parallel to the slope face.

Final Answer:
All the above