A rollback plan, also known as a backout plan, is a strategy designed to reverse changes made during a project in case of failure or undesired results1. A rollback plan is a form of risk response that allows the project to restore the previous state and minimize the impact of the issue. A rollback plan is usually created during the planning phase and implemented during the execution or closing phase of the project. The other options are not correct because:
A compliance check is a process that ensures that the project meets the relevant standards, regulations, and requirements. A compliance check is not a risk response, but rather a quality control or assurance activity.
A release plan is a document that outlines the scope, schedule, and resources for delivering a product or service to the customer. A release plan is not a risk response, but rather a communication or scope management tool.
A validation check is a process that verifies that the project deliverables meet the customer's needs and expectations. A validation check is not a risk response, but rather a quality control or assurance activity. References = CompTIA Project+ Study Guide: Exam PK0-005, 3rd Edition, Chapter 10:
Executing and Closing Projects, page 314; CompTIA Project+ Certification Exam Objectives, Domain
3: Project Execution, Objective 3.2: Given a scenario, execute and monitor project tasks using traditional methodologies; Backout Plan - DevX

Application deployment typically involves making a software application available for use, often involving downtime or unavailability of the application. The description of the project manager notifying users about the application being unavailable for a certain period aligns with standard practices during deployment phases. The other options, such as rollback plans, validation checks, and maintenance window schedules, although important, do not directly indicate the action of making an application available for use, as described in the scenario.

Meeting with the team to emphasize the importance of following change management procedures is the best approach to prevent future unauthorized changes. CompTIA Project+ highlights the importance of adhering to formal change control processes to ensure project alignment and accountability.

Smoke testing is a preliminary test to check whether the most essential functions of the software work correctly. It is often the first step in the testing phase and helps confirm that the build is stable enough for further testing. This aligns with CompTIA Project+ practices for software testing phases.

Meeting minutes should be prepared to ensure team members who are absent remain informed about assignments. Meeting minutes document what was discussed, decisions made, and assignments given during a meeting, providing a record of the team's progress and ensuring absent members are up-to-date. References:
CompTIA Project+ Study Guide Section 4.2.2
The project manager should prepare meeting minutes to ensure team members who are absent remain informed about assignments. Meeting minutes are a written record of what was discussed and decided in a meeting. They usually include information such as attendees, agenda items, action items, decisions made, issues raised, and deadlines assigned. Meeting minutes can help to communicate important information to team members who could not attend the meeting and provide a reference for future follow-up.