Answer:
The answers that seem to be correct are *Mention something you liked about the interview and *Use a business letter format. This would be a polite and friendly kind of a feedback mixed with good professionalism.
Explanation:
Now let's see why we rejected the other answer options.
1st of all, Sending a separate letter to each interviewer might be difficult as it could be really difficult to find the contact information of all the interviewers.
Sending thank-you letters to prospective co-workers is not a appropriate thing as you have not been selected still for the job.
Sending a quick text to the interviewer might not be appropriate as well. This is because those who interviewed you during the process are much more experienced and qualified than you and you should maintain your relationship with them professionally and with the utmost respect.