ANS:B - Flexible joint

The most suitable joint for pipes laid submerged under water, especially in areas where the bottom of the river bed is uneven, is a flexible joint. Explanation:

1. Flexibility: Flexible joints allow for movement and flexibility in the piping system, accommodating changes in alignment and position due to uneven terrain or settling of the river bed. They can withstand lateral and angular movements, as well as thermal expansion and contraction, without causing stress or damage to the pipes.
2. Sealing: Flexible joints provide a reliable seal against water ingress even when subjected to movement or changes in position. This is crucial for submerged pipelines to prevent leakage and maintain the integrity of the system.
3. Adaptability: Flexible joints can adapt to variations in the river bed's surface, including irregularities or uneven terrain, without compromising the structural integrity of the piping system. This adaptability ensures stable and secure connections between pipe sections, even in challenging underwater environments.
4. Types: Flexible joints for submerged pipelines may include various designs such as rubber expansion joints, mechanical couplings, or flexible pipe connections. These joints are specifically designed to provide flexibility and resilience in underwater applications, ensuring long-term performance and reliability.

In contrast, collar joints, flanged joints, and expansion joints may not provide the necessary flexibility and sealing required for submerged pipelines laid in uneven river beds. Collar joints and flanged joints are more rigid and may not accommodate movement or changes in position, while expansion joints are designed primarily to absorb thermal expansion and contraction rather than lateral movements or irregularities in the river bed. Therefore, a flexible joint is the most suitable choice for submerged pipelines in such conditions.