(c) is, as far as i can tell grammatically correct (although the commas are superfluous in such a construction), but it's simply absurd. the use of "or" here creates a sentence that is weirdly self-contradictory.
the clear point of the sentence is that the schools don't have enough computers, so you need to select the choice that's grammatically correct but also in line with that meaning. (d) is the only choice that satisfies both criteria.