Answer: They're all the same
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Reason:
 means we have 6 copies of "2" multiplied out as shown in choice B. That explains how A and B are the same, and we can say
 

The parenthesis are optional, but I find they're handy to count the '2's easier.
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Now notice that 

So,

The last step is possible because we have two copies of 
 multiplied together.
This shows that choice C is equivalent to A and B.
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Lastly,

The jump to the last step is possible because we have three copies of 
 multiplied together. 
This shows choice D is equivalent to the others. 
All four expressions are the same.
They represent different ways to say the same number. That number being 64.