Question

What forced Europeans to look for other routes east?

Land routes were closed off by Ottoman Turks.
Europeans were conquered by Ottoman Turks.
Europeans lost interest in Eastern products.
Land routes no longer led to needed goods.

Answer 1

"Land routes were closed off by Ottoman Turks."

After the Ottoman rise (notably the fall of Constantinople in 1453) overland trade routes to Asia became more difficult, expensive, and controlled by Muslim intermediaries, prompting Europeans to seek new sea routes to the East.