a. Europeans had begun to demand valuable goods such as silk and spices that could only be found in Asia.
d. The Ottoman Turks controlled the Silk Road, which was the main land route from Europe through the Middle East to Asia.
The Ottoman Turks were involved in trade and merchant traffic themselves, and they often charged high rates for travel through the territories they now controlled. Also, as religious and cultural opposites, European Christian culture and Islamic culture often came into conflict. So finding an alternate route was appealing to Europeans who sponsored exploration by sea, especially as demand for goods from Asia increased in Europe. The Europeans also had goods to exchange in Asia, such as animal skins and furs ... and also slaves. There wasn't really a market of "manufactured goods" back in the 15th century, since we're in pre-industrial times in that era.
During the fifteenth-century, Europeans weren't just out to search for a sea route to Asia, they had to search for one since the Ottoman Turks controlled the Silk Road, which was the main land route from Europe through the Middle East to Asia. Europeans had began to demand valuable goods such as silk and spices that could only be found in Asia since it has been quite useful to their daily living. Of course, in turn, European merchants had manufactured goods to sell in Asia for those valuable goods such as silk and spices, but when the land route wasn't available and safe anymore, they had to search for sea routes.
