IMO "E" will be wrong on GMAT for SV agreement. 'they' also creates confusion about whom it is referring to.
Folks, be careful with Barrons. My suggestion, just ignore this & go ahead.
while I agree that Barron isn't the best source, it is very arrogant to say that one should ignore it or this question in particular... there are tons of questions in this forum that don't state the source but are from even worse sources... and there are plenty of such questions in SC 1000 document, which also lucks explanations!!!