The correct answer is - to expedite trade by finding and charting alternative methods of transport to Asia.
The Ottomans made a large empire, and they managed to gain control of the trade routes between Europe and Asia, demanding high taxes for both sides if they wanted to use their territory for trade. The Europeans did not fancy this, so they sponsored an expedition, one of many to come, so that a new sea route is found for Asia, thus trading through it freely. Columbus was the main explorer of one of those missions. He went westwards with the ships he had at disposal, hoping that he will come to the eastern coast of Asia. He indeed came to some islands, and later a mainland, and all excited about it he called the natives Indians, as he actually thought it is India that he reached. Little did he knew that he actually discovered new continents.