What was Augustus's (r. 27 BC - 14 AD) original name before becoming emperor?

Octavian is correct. Born Gaius Octavius Thurinus, he was posthumously adopted by his great-uncle Julius Caesar and took the name Gaius Julius Caesar Octavianus; historians call him “Octavian” for the years before the Senate granted him the honorific “Augustus” in 27 BC. The other options are different figures: Tiberius was his successor, “Julius” refers to Caesar himself, and Caesarion was Cleopatra and Julius Caesar’s son. Context: As Octavian, he defeated Mark Antony and Cleopatra at Actium, ending the civil wars and ushering in the Principate and the Pax Romana. Memory tip: O comes before A—Octavian before Augustus—just as his earlier name came before his imperial title.