What was a key discovery during the early sixteenth century that made regular trade possible across the Atlantic ocean?

Question

A.Square sails

B.Longitude

C.The gulf stream

D.Monsoonal winds

I think the answer is D. Monsoonal winds.

Answer 1

square sails were invented long before 1500

longitude by chronometer was invented by John Harrison in late 1700s

The Gulf stream allows you to sail across to America on southern route with the current and return further north with the winds from the west.
read this:
The Gulf Stream was first reported to the Western world by the explorer Ponce De Leon around 1513. Making use of it and the Westerlies, Spanish ships were able to navigate home faster, improving their ability to trade and colonize North America and the Caribbean region. In the late 18th century, American innovator Ben Franklin mapped the Gulf Stream, eventually convincing British sea captains to make use of the beneficial current to cut return trips to Europe by days and even weeks.

I do not associate monsoons with the North Atlantic Ocean.

Answer 2

That is from:
http://www.wisegeek.com/what-is-the-gulf-stream.htm