A) The original looks pretty good here. Compensating for the lack of oxygen correctly modifies Tibetans produce boosted supplies, and the semicolon effectively connects the two clauses. There is, however one error in this choice. The singular, bloodstream, seems to convey that all of the Tibetans share one bloodstream.

B) The placement of which in this version causes it to read as if their bloodstreams, rather than nitric oxide, allow blood vessels to expand. Also, the singular, bloodstream, seems to convey that all of the Tibetans share one bloodstreamn.

C) This version conveys that the researchers rather than the Tibetans compensate for the lack of oxygen.
That meaning does not make sense. Also, the singular, bloodstream, seems to convey that all of the Tibetans share one bloodstreamn.

D) This one conveys that the Tibetans are somehow "the producers of boosted supplies of nitric oxide". So are they the only producers? What does that even mean? Also the placement of according to researchers is not optimal, creating an awkward second clause.

E) This version once again conveys the illogical meanings that the researchers compensate for the lack of oxygen and that the Tibetans share one bloodstream. The use of present perfect have found in this choice creates a false decision point in that a similarly incorrect choice, C, says merely found. Both choices are incorrect for various reasons, and so this distinction is irrelevant.

So even though the use of the singular, bloodstream in choice A renders it slightly incorrect, because overall its structure is effective and conveys a logical meaning, A is the best answer.