The correct option is A two weeks'
The sentence that includes blank 5: "The doctor has advised ____5___ rest to help me recover fully from the sickness." forms part of the main body.
Option A: As mentioned in the description before the letter, "the doctor has advised him to take two weeks' rest", A mentions the correct number of weeks advised by the doctor. Hence A is correct.
Option B: 'Some' is a non specific number, whereas in the description of the letter, a definite number is given: two weeks.
Option C: Similarly, 'one day's' is not mentioned in the description. Hence options B and C are incorrect.
Option D is incorrect because the answer lies in A.