Constantine moved the capital of the Roman Empire to Constantinople primarily because a) it was an ideal location for trade and c) the geography made it easy to defend.
Constantinople (modern-day Istanbul) was strategically located between Europe and Asia, making it a significant hub for trade. Additionally, its location on the Bosporus Strait provided natural defenses against invasions. While the option b) suggesting it was the only city left in the Western Roman Empire is inaccurate, the other two reasons are valid motivations for Constantine's decision.