Raveling in bituminous pavement is the phenomenon of the disintegration and separation of aggregate particles from the bitumen binder. It leads to the loss of surface integrity and the formation of loose gravel on the pavement surface. Among the given options, the correct answer is option 'D' - low bitumen content.

Here is a detailed explanation of why low bitumen content can cause raveling in bituminous pavement:

1. Importance of Bitumen: Bitumen is a highly viscous binder that holds the aggregate particles together in bituminous pavement. It provides adhesive properties, ensuring that the aggregates are well-coated and bonded. The proper amount of bitumen is necessary to maintain the stability and structural integrity of the pavement.

2. Insufficient Binder: Low bitumen content means that there is not enough binder to adequately coat and bind the aggregate particles. This results in poor adhesion between the aggregates and the bitumen. As a result, the aggregates become more susceptible to dislodgement and separation.

3. Lack of Cohesion: The cohesive strength of the bitumen is crucial to resist the shear forces and traffic loads imposed on the pavement. Insufficient bitumen content leads to a decrease in the cohesive properties of the mixture. This lack of cohesion allows the aggregate particles to detach from the bitumen film, leading to raveling.

4. Increased Porosity: Low bitumen content can also result in increased air voids within the pavement mixture. These voids act as pathways for water infiltration, which can further deteriorate the bond between the aggregates and the bitumen. The presence of water can accelerate the raveling process by weakening the adhesive properties of the binder.

5. Traffic and Environmental Factors: Raveling is often exacerbated by traffic loads and environmental conditions such as freezing and thawing cycles. The absence of sufficient bitumen content reduces the pavement's ability to withstand these external factors, leading to accelerated raveling.

In conclusion, low bitumen content in bituminous pavement can cause raveling due to poor adhesion, decreased cohesion, increased porosity, and reduced resistance to traffic and environmental factors. It is essential to ensure proper bitumen content during the design and construction of bituminous pavements to prevent such distresses and maintain the longevity of the pavement.