Answer:
a. Prior period adjustments.
Step-by-step explanation:
"Retained earnings is the cumulative total of earnings that have yet to be paid to shareholders. These funds are also held in reserve to reinvest back into the company through purchases of fixed assets or to pay down debt."
Prior period adjustments in the beginning balance are key to calculate the retained earnings at the end of the period:
Retained Earnings = RE Beginning Balance + Net Income (or loss) – Dividends.
Therefore, prior period adjustments may either increase or decrease RE.
Reference: Morah, Chizoba. “Which Transactions Affect Retained Earnings?” Investopedia, Investopedia, 11 July 2019