In a software development project with an external customer, the project manager (PM) is responsible for coordinating and managing the project-including building, maintaining, and controlling the schedule.
CompTIA Project+ specifically identifies schedule management as a core skill area (e.g., maintaining schedules and related activities), which aligns directly with option A.
The other options describe responsibilities typically owned by other roles or governance functions. Defining functional requirements is usually led by a business analyst and/or product owner/customer stakeholders, with the PM facilitating requirements activities and ensuring traceability and impact awareness-not owning definition outright. Developing software components is a developer/engineer responsibility, not the PM's.
And assessing/authorizing security policies is commonly handled by security/compliance leadership (or a governance authority), while the PM ensures the project aligns with required policies and coordinates related work.
Therefore, the clearest PM responsibility in this scenario is managing the project schedule (along with integrating scope, resources, risks, stakeholders, and communications).