Answer:
- "The clients have acknowledged the email and are pleased with our work" is formal.
- "We regret to inform you that your application has been rejected" is formal.
- "I appreciate your attention" is formal.
- "Thanks for listening to my story" is informal.
- "The computer's been making noises forever" is informal.
- "Stella's team will be going up against Tom's team for the sales pitch" is informal.