Answer:
a) No change is needed.
Step-by-step explanation:
The sentence shows a sequence of actions, one happening after the other, and as so it requires no pauses. In other words, there is no need for a semicolon, a comma or a colon to be added to the sentence, as it is already complete and correctly punctuated the way it is. Moreover, the two verbs in it ("had" and "decided") have a single subject: "my sister", which means all that appears after the subject is part of the same predicate.