I'm not personally a big fan of this kind of exercises (the one mentioned in the question). It's too often that the context wasn't clear enough.
However, let's analyze it. Here are the definitions of those keywords I chose from what Google gave,
blaze (v.) set an example by being the first to do something; pioneer. "small firms would set the pace, blazing a trail for others to follow"
courage (n.) strength in the face of pain or grief.
chivalry (n.) the combination of qualities expected of an ideal knight, esp. courage, honor, courtesy, justice, and a readiness to help the weak.
Here are some of the definitions for pioneer,
- a person who is among the first to explore or settle a new country or area.
synonyms: settler, colonist, colonizer, frontiersman, frontierswoman, explorer, trailblazer, bushwhacker
"the pioneers of the Wild West"
- a person who is among the first to research and develop a new area of knowledge or activity.
"a famous pioneer of birth control"
synonyms: developer, innovator, trailblazer, groundbreaker, spearhead;
founder, founding father, architect, creator
"an aviation pioneer"
Obviously, pioneer is a synonym for settler, colonist, explorer, and innovator. And innovator is not very far from inventor. However, let's get back to the sentence,
The pioneers left a blazing trail of courage and chivalry.
With the words courage and chivalry, the word inventor is an unlikely answer (though it can be argued that many inventors were also full of courage and chivalry qualities). The basic sense of the word inventor is "a person who invented something." Also, the word colonialist is not quite the same as colonist. The Free Dictionary defines colonialist as a believer in colonialism. This makes colonialist not a good answer.
Thus we are left with two good choices: explorers and settlers. Here are their definitions,
explorer (n.) a person who explores an unfamiliar area; an adventurer.
settler (n.) a person who settles in an area, typically one with no or few previous inhabitants.
When talking about leaving something behind (a blazing trail in this case), settler seems to fit better. However, in my opinion, both of them (explorers and setters) can be a good answer.