In a study of office workers at a corporation, Australian researchers

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­Thank you! Edited the tag.

C should be the answer.
The conclusion is "work breaks ( can be any sort I.e going for a tea break or surfing internet etc ) are rejuvenating .
The premise cited is :- One example where the workers browsing internet proved more productive than the workers who abstained from internet.
Option C strengthens by saying that the workers who abstained from internet not just stayed away from the internet but dint indulge in too many other forms of breaks also. They took far fewer breaks. And eventually they proved less productive. Hence the conclusion i.e "work breaks are helpful in increasing productivity and rejuvenating " gets strengthened .

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GMATNinja WHY IS OPTION B WRONG­

 

­The researcher's argument is that "frequent work breaks are rejuvenating and improve concentration, thereby increasing productivity".

B) States "Those who took work breaks to browse the Internet were more productive than those who took equally frequent breaks away from the computer"

This does not strengthen the researcher's argument, in fact it undermines their argument since it implies that work breaks are not the cause of increased productivity, rather specifically breaks to browse the internet increase productivity. 

My advice here would be to carefully identify the researcher's argument word by word and evaluate the answers precisely against this.

­Understanding the argument - ­
In a study of office workers at a corporation, Australian researchers found that, on average, those who spent up to 20 percent of their work day browsing the Internet for purposes that were not work-related were 9 percent more productive per hour of actual work than those who completely abstained from going online at work. - Background info. It mainly focuses on online breaks and their effects. 
The researchers concluded that frequent work breaks are rejuvenating and improve concentration, thereby increasing productivity. - Conclusion. The conclusion makes a giant leap to "frequent work breaks" that may include non-online related breaks, which is a gap. 

Which of the following pieces of information about the workers studied would, if true, most strengthen the researchers’ argument?

A. Those who were the most productive had jobs that required them to use the Internet frequently for work-related purposes. - Weakener. Reverses the cause-effect relationship. 

B.  Those who took work breaks to browse the Internet were more productive than those who took equally frequent breaks away from the computer. - This is not the gap. Distortion. 

C.  Those who abstained from going online during their work day generally took far fewer work breaks than the other workers. - Yes. So overall, they took a few breaks. The argument covers online work, but this option covers the gap in general work breaks. Ok. 

D.  Those who were the most productive relative to their colleagues tended to take longer work breaks than the least productive of their colleagues. - The argument already shares this. Moreover, "longer work breaks" or shorter breaks are out of scope. The main idea is whether they take frequent breaks or not. Meaning whether they take 3 or 4 or more breaks. The issue is not whether they take 30 min or 1-hour break. 

E.  Those who spent more than 20 percent of their work day online were less productive, on average, than those who abstained from going online.­ - Weakener. ­