The correct option is
A Content of the letter
Here, we have to explain the format of the letter. The 22nd blank in the letter is the place where we actually write the body of the letter. Let's have a look at the options.Option B - 'Receiver's address' is not needed in an informal letter as we usually write informal letters to our friends and family members. The 22nd blank is for the body of the letter. Hence, option B is incorrect.
Option C - 'Salutation' is written right after the address and the date. A salutation is used to address the reader. It is written before the body. Hence, option C is incorrect.
Option D - The date of writing is written right after the sender's address. Hence, option D is incorrect.
Option A - Yes, the 22nd blank is for the content or the body of the letter. Here, the sender can actually write the matter of the letter. Hence, option A is correct.