I would go with <em>b. more efficient ships</em>.
Mechanical motors were very helpful, they helped the ship move, but I don't believe they were the development that aided the explorers the most.
Sure, they could've made maps, but what was the point? If they already knew the layout of the oceans and lands, then they could've easily traveled wherever they pleased without trouble. This also applies for the spherical globe.
A greater supply of efficient ships helped sailers get by tougher tides and harsher weathers. Ships were produced to be better, stronger, and more durable. They were able to handle more weight and carry more passengers.
Furthermore, the production of efficient ships were the development that most aided explorers in their ability to sail the oceans to discover the New World during the Age of Discovery.
(This is all from my logic. I've not read your lesson, therefore, I do not truly know the answer other than using my common sense.)
Hopefully, this helps.
