I go with A.
C ends with "remains" which I'm assuming changes the meaning of the sentence. A's "had remained" means they built the houses on it and is the intention of the original sentence (IMO.)
Also, something about "one of the only eight" doesn't work for me. I'm not up on all my idiom rules but isn't it "one of only eight"??
You are right Jumbee. The sentence is in the past tense and "had remained" is necessary to convey the idea that the lands no longer remain... in 1989 it had remained undeveloped but no longer today since it's already being built on nowadays.