The app builder should use the CASESAFEID(Id) formula to create this new field. The CASESAFEID function returns the 18-digit case-insensitive version of the record ID, which is unique for each record in Salesforce. The 18-digit record ID is useful for exporting data to external systems that require case-sensitive IDs1. Option A, C, and D are not formulas that can return the 18-digit record ID.

An Approval Process is the appropriate functionality to ensure all free parts receive the marketing director's sign-off. It allows for setting up a process for record approval.
References:
Salesforce Help - Approval Processes

Record-triggered flow is the automation tool that the app builder should use to best address all these requests of sending an email to the finance manager and creating a task for the sales manager when a new client is won. According to the Salesforce documentation, "Record-triggered flows automate processes for you. They start when a record is created or updated, or when a platform event occurs." A record-triggered flow can perform actions such as sending an email alert or creating a record based on criteria or conditions. Screen flow, Apex, and Approval process are not automation tools that can handle all these requests.