The correct option is B Subject
Formal letters are usually written in this format:
The sender's address,
Date.
The receiver's address,
Salutation
Subject (sometimes the subject is not included)
Body of the letter
Appropriate close (Thank You)
Closing salutation (Yours sincerely)
Name and signature
Title (if applicable)
Option C: The subject is written after the salutation, as mentioned in the format. It means that the subject is in blank no. 24, since this is the only option that is written directly after the receiver's address. Hence from these options, C is correct.
Option A: The sender's address is written in blank 21, and not in 24. Hence option A is incorrect.
Option B: The receiver's address is written in blank 23 (after the date), hence B is incorrect.
Option D: The date is written in blank 22, hence D is also incorrect.