From this sentence, we know that some people helped the rebels even though they weren't rebels themselves. The word describing those people is needed for the first blank. For the second one, we need a word describing how this act of help could be perceived.
Out of the possible answers, only one first word is somewhat opposite to "rebels" - it's choice (C), "nonpartisans." "Nonpartisan" means "not supporting one political group over another" and it fits in the first blank. The second word in this answer, "seditious," means "encouraging people to rebel," so it's also correct. Among the other second words, we might be tempted to use choices (A), "treasonous" (trying to overthrow a government or otherwise betray it), or (E), "subversive" (systematically trying to undermine a government working secretly from within). However, the first words in those answer choices don't fit.
The correct answer is (C).