Definition of Slope Failure:
Slope failure refers to the downward movement of a mass of soil or rock along a surface of sliding. It is a common phenomenon in geotechnical engineering and can result in significant damage to infrastructure and pose risks to human safety.

Types of Slope Failure:
There are several types of slope failures, including base failure, slope failure, and face failure. Each type is characterized by the location of the failure surface and the mechanism of failure.

1. Base Failure:
- Base failure occurs when the failure surface intersects the slope at its toe, which is the bottommost part of the slope.
- This type of failure is typically associated with the loss of support at the base of the slope, leading to a sliding movement along the base.
- It is also known as toe failure or sliding failure.

2. Slope Failure:
- Slope failure refers to the failure that occurs along a surface of sliding that does not intersect the slope at its toe.
- In this case, the failure surface is located within the slope, and the failure mechanism can involve sliding, toppling, or a combination of both.
- Slope failure is often triggered by factors such as excessive slope steepness, weak soil or rock materials, and changes in groundwater conditions.

3. Face Failure:
- Face failure occurs when the failure surface intersects the slope at its face, which is the exposed surface of the slope.
- This type of failure is commonly observed in slopes with steep faces, such as cut slopes in road or railway construction.
- Face failure can result from various factors, including weathering and erosion, which weaken the slope face and promote its failure.

Correct Answer:
The correct answer to the given question is option 'B' (Slope failure) because when the failure occurs along a surface of sliding that does not intersect the slope at its toe, it is known as slope failure. Base failure and face failure have specific locations of the failure surface, whereas slope failure encompasses failures that occur along any surface within the slope.