Not necessary, because if it were not true — if a demagogue COULD get public support to topple a regime before a comprehensive system of general education is in place — that supports the author’s argument. The author’s position is that demagogues could topple a harmless regime. The negation of (A) doesn’t undermine that position or the support for it.

The author said that literacy informs “more” people of injustices. That doesn’t commit the author to thinking that people can’t generally be aware of injustice without literacy. Maybe they can be — it’s just that more people would be aware if literacy spread.

Not necessary, because the author’s argument concerns what makes a benign regime vulnerable. The author isn’t committed to any position regarding what would tend to preserve a benign regime.

Necessary, because if this were not true — if lack of a general education does NOT affect ability to differentiate between legitimate and illegitimate calls for reform — then we have no reason to think that literacy will make people vulnerable to demagogues. The negation of (D) allows for the possibility that people without general education can recognize illegitimate calls of reform from demagogues and won’t necessarily be vulnerable to them.

The certainty is too extreme here. The author never argues that a regime “will be toppled” by a demagogue. The conclusion is merely that some benign regimes “may” be toppled.