This question tests you on your ability to work with math exponents. Do you understand the rules and do you know which rules to use in which situations?
Well, in this case, they present two large exponents. You are subtracting these large exponents - both of which can be reduced to base 2. Note that you should not be mixing the popular "keep the base, add the exponents" rule in this case because we are not dealing with multiplying to exponents with the same base. Rather, we are dealing withe addition/subtraction of tow exponents so this common rule of thumb does not apply.
You'll need to recognize that the 8 base can be converted to a base 2 and that you would need to multiply the existing exponent by a certain amount so that you can combine this term with the second term.
Once you've simplified everything down, you'll need to break down one of the numbers into prime factorization factors and find which one is the highets. That's your answer.
source: GMAT Pill
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