I believe it would be "One hundred days" as the "Full period" part emphasizes the full length of the period of time that is "One hundred days", thus influencing my opinion and idea of the answer.
A pronoun antecedent agreement has to do with keeping the consistency and inclusion of genders or lack thereof when writing. For example if you were to say "A student must call their parent/guardian before riding a different bus or she will not be allowed to ride said bus." you would be violating the pronoun antecedent agreement by saying she rather than they.
Answer:
to is the correct answer here in this sentence
