Assuming that you are required to produce a Statement of Financial Position at the end of the period, then you use closing balances at the end of the period.
(We stopped using the term ‘Balance Sheet’ many years ago – it is called the Statement of Financial Position. Also, you cannot be asked to produce a full SOFP in the exam – only extracts from it.)
I suggest that you watch my free lectures. They are a complete free course for Paper FA and cover everything needed to be able to pass the exam well.)