"None of them is" vs "none of them are" is a heated debate among grammarians (at the 1993 grammar symposium in Berne, Switzerland, two noted British scholars were stabbed by an angry Australian over just this issue).

The traditional rule is that, since "none" is a contraction of "no one", and "one" is singular, none always takes a singular verb, regardless of what follows.

In popular usage, however, plural verbs are often used (most people find them more natural in the construction).

Today, at least in the regular world, both are considered acceptable. However, we're not dealing with the real world, we're dealing with the wild and wonderful world of the GMAT, which brings us to your question: "which one does the GMAT accept"?

I'll do a quick flip through the OG on Monday (I'm not in the office until then) and see if it arises in any questions - I'll also talk to the Kaplan team about it.

I just gone through all OG guides, there is no tests of none, i mean no Q tests the singularity/plurality of none...IMHO Manhattan is ridiculous with certain pronoun tests to the extent they rely on it...