Why was control of constantinople important

Fall of Constantinople, (May 29, 1453), conquest of Constantinople by Sultan Mehmed II of the Ottoman Empire. The dwindling Byzantine Empire came to an end when the Ottomans breached Constantinople’s ancient land wall after besieging the city for 55 days. Mehmed surrounded Constantinople from land and sea while employing cannon to maintain a constant barrage of the city’s formidable walls. The fall of the city removed what was once a powerful defense for Christian Europe against Muslim invasion, allowing for uninterrupted Ottoman expansion into eastern Europe.

i would think the answer is between b and d i would say d sorry if wrong