The main motivating factor for Christopher Columbus's investors, specifically the Spanish monarchy, was to gain access to lucrative trade routes and resources. Columbus was primarily interested in finding a westward route to Asia to access the spice trade, which was highly profitable at the time. Meanwhile, Vasco da Gama, who was later sent by Portugal, aimed to reach India by sailing around Africa to establish direct trade routes for spices and other goods, thus gaining access to both the spice and silk trades.
Among the options you provided, the closest to the historical motivations would be:
"Columbus wanted to gain access to the spice trade and da Gama wanted to gain access to the silk trade."
Both explorers aimed to expand their respective countries’ trading empires and claim glory for their discoveries.