This is the best option because it addresses the root cause of the problem, which is the unclear product requirements. By analyzing the problem with both teams, the project leader can facilitate communication, collaboration, and alignment between them. This can help to clarify the product vision, prioritize the backlog, and identify any gaps or errors in the work done so far. This can also help to improve the quality of the product and the satisfaction of the stakeholders.

The other options are not as effective because they do not address the underlying issue or they may create more problems. For example:

B. Selecting new team members may disrupt the existing team dynamics and introduce more uncertainty and confusion. It may also delay the project and increase the cost.

C. Demanding that the business team define final requirements may create conflict and resentment between the teams. It may also be unrealistic or impractical to expect final requirements in a complex and changing environment.

D. Telling the teams to continue following the initial plan may result in wasted time and resources, as well as low-quality outcomes. It may also ignore the feedback and learning from the previous sprints.