The apostrophe marks possessive case. (belonging to)
The leg of the man is broken. == The man's leg is broken. (One man has a leg = man's leg)
The dresses of the girls are pretty.
The girls' dresses are pretty. (Many girls have dresses == girls' dresses)
There is one today, so (a) is correct. The letter of today is B!
There can be more than one week.
The letter of this week is B!
This week's letter is B.
The letter of the next 3 weeks is B!
The next 3 weeks' letter is B!