the entire sentence after "that" is in fact a clause, so there's no problem here. the GMAT won't test you on whether to put a comma after "that", so no need to worry about that. the comma that is there is because "when speaking" is an introductory modifier to "individuals"..

_________________
Tim Sanders
Manhattan GMAT Instructor

I have question about the clause that modifies the blind person:
"thus have never seen anyone gesture".

Should the gesture be gestured or gesturing?

I saw an earlier post on this question but not so satisfied with the example posted.

I saw John run to the class.

Here run is past participle to modify John. It is no doubt.

However gesture, the past participle should be gestured....
Why does the "gesture" which modifies the singular pronoun stay as it is without any ing or ed?

thanks Tim. that makes sense. Am i correct about why this could be wrong?

"Research has shown that, when speaking, individuals..."

when speaking could modify "has shown"... as a subject/verb modifier.. but who is speaking? there are no researchers mentioned. ends up as kind of nonsense either way.

"Research has shown that when speaking, individuals..."

"when speaking" should modify the verb performed by individuals.. this happens to be "make hand gestures"

first --
OFFICIALLY CORRECT ANSWERS ARE CORRECT!
do not question officially correct answers!
far too many students on this forum make the mistake of questioning the correct answers; please note that doing so is a complete waste of your time and effort. i.e., exactly 0% of the time that you spend posting "isn't this official answer wrong?" is productive, and exactly 100% of that time is wasted.

"is this correct?" is NEVER a productive question to ask about one of GMAC's correct answers -- the answer is always yes.
"is this wrong?" / "is this X type of error?" is NEVER a productive question to ask about one of GMAC's correct answers -- the answer is always no.

instead, the questions you should be asking about correct official answers, if you don't understand them, are:
"why is this correct?"
"how does this work?"
"what understanding am i lacking that i need to understand this choice?"

this is a small, but hugely significant, change to your way of thinking -- you will suddenly find it much easier to understand the format, style, and conventions of the official problems if you dispose of the idea that they might be wrong.



despite your apparent confidence that there is “no doubt” here, this is incorrect. that is not a past participle; it's basically an infinitive form, without “to” in front.
other examples:
i helped my wife wash the dishes
caffeine can help you be aware of your surroundings if you are sleepy
try substituting past participles (“washed” and “been”) for these two boldfaced words; you will notice that the result is absurd.