The project lead should have involved the operations stakeholders during the planning phase of the project.
This would have ensured that the system was designed to meet the day-to-day requirements of the operations team. Additionally, it would have allowed for the identification of necessary support hours and resources. By involving operations stakeholders early in the project, the project lead could have avoided the issues that arose after the project was handed over to operations. References: Project Management Professional (PMP) Examination Content Outline, June 2015, PMI; A Guide to the Project Management Body of Knowledge (PMBOK Guide) - Sixth Edition, Project Management Institute, Inc., 2017.

According to the PMBOK Guide, the project manager should analyze the impact of the developers' resignation on the project scope, schedule, cost, quality, and risk, and inform the relevant stakeholders about the situation and the possible options to address it. This is part of the Manage Team process, which involves tracking team member performance, providing feedback, resolving issues, and managing changes to optimize project performance. By analyzing the impact and informing the stakeholders, the project manager can facilitate collaborative decision making and ensure alignment of expectations and objectives. References:
(Professional in Business Analysis Reference Materials source and documents) PMBOK Guide, 6th edition, Chapter 9, Section 9.4, Manage Team PMP Exam Prep, 10th edition, Rita Mulcahy, Page 357, Manage Team

The project manager should request the project sponsor prioritize and if necessary, allocate additional budget so the team can work on a single project. This is because working on multiple projects at the same time can reduce the team's focus, productivity, quality, and morale. It can also increase the risk of delays, errors, conflicts, and rework. Agile approaches require dedicated and cross-functional teams that can deliver value in short iterations. Therefore, the project manager should seek the support of the project sponsor to ensure that the team has the necessary resources and time to work on one project at a time. Providing an incentive plan so the team members will work more unaccounted hours on multiple projects (option B) is not a good solution, as it may lead to burnout, stress, and dissatisfaction among the team members. It may also violate the labor laws and ethical standards. Brainstorming with the team members on how they can deliver on multiple projects within the given budget (option C) is a possible option, but it may not be realistic or feasible, as it may compromise the quality and scope of the projects. Instructing the team members to spread evenly across projects so mistakes will not occur within the current budget (option D) is not a good option, as it may create confusion, duplication, and inconsistency among the team members. It may also prevent the team from forming, storming, norming, and performing as a cohesive unit. References:
PMBOK Guide, 6th edition, page 52-53, section 2.4.2 Project Team
[Agile Practice Guide], page 26-27, section 3.1.1 Team Composition
PMP Exam Prep, 10th edition, page 455, question 455, answer and explanation

According to the PMBOK Guide, the resource calendar is a component of the resource management plan that identifies the availability, capability, and skills of human resources, as well as the availability of physical resources. The resource calendar may be affected by various factors, such as organizational policies, holidays, vacations, and work shifts. If the project team members enroll in the work-from-home policy, the project manager should update the resource calendar to reflect their availability and working hours, as well as any potential impact on the project schedule, budget, quality, and risk. The project manager should not update the communication styles, roles and responsibilities, or stakeholder engagement plan, as these are not directly related to the change in the resource availability. References: PMBOK Guide, 7th edition, pages 309-310, 313-
314.