The correct option is
B Today's date
Formal letters end with the name of the person who has written the letter, i.e., Ramesh. After the name and designation (if applicable), the letter ends.
So, from the options, none of the dates will be used.
The birth date is irrelevant in this letter. It is not a part of the letter format. Hence, it is not added.
The day's date is also not required, as letters usually don't close with dates.
'Start date' is also the same as 'today's date', which is not necessary. (Start date refers to the day when an activity begins.)
Hence, options A, B and C are incorrect.
Since none of the above options are correct, option D: none of the above, is the correct one.