The correct option is
B Date
Here, we have to fill in the first blank of the given letter. Before the blank, the address of the sender is written. Hence, we can say that the sender must write the date of writing after his address. Let's look at the options.
Option A - 'Receiver's address' is not needed in an informal letter as we usually write informal letters to our friends and family members. As this letter is written by Raj to his father, it is an informal letter. Hence, option A is incorrect.
Option C - Time is not needed in an informal letter. It isn't included in the format. Hence, option C is incorrect.
Option D - 'Subject' is written in a formal letter only. Since, this is an informal letter, 'subject' will not be written. Hence, option D is incorrect.
Option B - Yes, after the address of the sender, we must write the date. Hence, option B is correct.