In a campaign, the volunteers who are in charge of going door-to-door in order to explain information about the candidate and issues are working as

a.

field workers

b.

fundraisers

c.

researchers

d.

coordinators

Well, they're not coordinating anything, instead they are directly engaging with people - they don't work on the large-scale, but small scale, so they want to make a small localized impact, rather than contribute to the big-scale planning. 

Also, they are not asking for money - they are just informing. If they were asking for money, they would be fundraisers. But since they're not, and since they're working locally, they are field workers. (a)