One report concludes that many schools do not have, or likely to have, enough computers to use them effectively.

The sentence means that many schools neither have enough computers nor they are likely to have many computers to use them effectively.
Notice what follows “or” is not grammatically parallel to what precedes it. X = many schools do not have (clause) and Y = likely to have (adverb + to verb). So we have parallelism issue here as well.
POE:
Choice A: Incorrect as discussed.
Choice B: Same parallelism issue as A.
Choice C: With the use of “or”, what this sentence now means is that the schools do not have many computers but are likely to have many computers. This is a suggestion made in a very awkward manner which is not the logical intended meaning of the sentence. It is rather the contradictory suggestion. Hence usage of “or” is incorrect here. So the idiom error leads to meaning error also.
Choice D: Correct answer, presents the logical intended meaning of the sentence.
Choice E: There are two negative words together in this choice.

1. First understand the logical intended meaning of the sentence. Then choose the answer choice that conveys that logical intended meaning.
2. In a parallel list what follows the marker should be logically as well as grammatically parallel to what precedes the marker.
Hope this helps.
Shraddha