This must be false. We know that all people who are able to fully concentrate are of above-average intelligence. So it's impossible for some people to be able to fully concentrate but be of below-average intelligence.

This could be true. We know for sure that most speed readers have an increased ability to concentrate; but we don't know whether they have the ability to "fully" concentrate, or whether they are of above-average intelligence. It's possible all who can speed read are of above-average intelligence.

This could be true. We know that all people who can fully concentrate are of above-average intelligence, but this doesn’t mean that everyone of above-average intelligence can fully concentrate. So there can be many people of above-average intelligence who can't fully concentrate.

This could be true. Although speed-reading is usually accompanied by "increased" ability to concentrate, that leaves open the possibility some people can speed-read but still have very little ability to concentrate.

This could be true. We know that being trained in speed-reading is usually accompanied by increased ability to concentrate. It's possible everyone who can speed read can concentrate at least somewhat. Most of them experienced increased concentration, but this leaves open the possibility that everyone can concentrate at least a little bit.