D is the correct answer, as the risk with this plan is that multiple work-streams with dependencies could impact the go-live. If the legacy system upgrade and the new integration are not aligned and coordinated, there could be delays, errors, or failures in the project delivery. A is incorrect, as the deadline being scheduled during a Salesforce release is not a risk, but an opportunity to leverage the new features and enhancements that Salesforce provides. B is incorrect, as the project team using the Waterfall methodology is not a risk, but a choice that depends on the project scope, complexity, and requirements. C is incorrect, as the legacy system team using an Agile methodology is not a risk, but a choice that depends on the project scope, complexity, and requirements. You can learn more about the risk management in the Project Management Strategies for Salesforce Implementations module on Trailhead.