Final answer:
Mark Antony was the co-ruler of the Roman east who was allied with Cleopatra. The correct option is b.
Step-by-step explanation:
The co-ruler of the divided Roman territory who ruled in the east and fell under the charm of Cleopatra, the queen of Egypt, was Mark Antony. He set up his court in Alexandria, Egypt, and forged both a political and romantic alliance with Cleopatra. They aimed to rule the eastern provinces in defiance of Octavian (the later Augustus). Mark Antony and Cleopatra declared their children as heirs to regions within the Roman territories, which antagonized Octavian, leading him to start a publicity campaign against Antony in Rome.
Eventually, Octavian declared war on Cleopatra's regime, and Antony was defeated at the Battle of Actium in 31 BCE. After their defeat, Mark Antony and Cleopatra in 30 BCE, paving the way for Octavian to become the undisputed ruler and the first Roman Emperor, taking the title Augustus. Hence the correct answer is b.