I've seen a few people recently ask on this forum whether "where" needs to refer to a physical location -- I'm guessing some prep material claims that's true -- and while "where" of course can refer to a location, it doesn't always. Like almost every common English word, it's versatile and can be used in lots of ways. If someone asks, say, "where did you read that?" the person isn't usually asking what city or library you were in. They're usually asking the source of your information. Or if someone says "I'm reading a novel and I don't know where the story is leading", that person isn't questioning what city the characters in the story are going to end up in. Or if someone says "I don't know where I'll be in five years", they occasionally might mean "I'm not sure what city I'll be in", but much more often they mean "I don't know what career (and other life situation) I'll be in five years from now." All of those uses of "where" are perfectly correct.
But in this question, "where" sort of does refer to a location anyway -- it refers, in the OA, to the "community", which you can think of as a location, even if the word carries other connotations. So the use of "where" is no reason to prefer D to B here. The meaning of D is a bit strange; if the community has "70% of the incomes dependent on remittances", then some other place (it's not clear where) has 30% of those incomes. It's like saying "The community has 70% of the doctors", which would mean some other place, perhaps the rest of the country, has the remaining 30% of the doctors. That's not what the sentence means to say -- it means to say that 70% of the incomes in the community depend on "remittances", while the other 30% of the incomes in the same community do not depend on those "remittances", which is the meaning B conveys.
I count at least four issues even with the "OA" here, though, so I don't think it's the best question to study in detail.