A. 2^9 / 2^7.....not this one...because when dividing by the same base, u keep the base and subtract the exponents.
B. (6^10)^-7...this one u multiply...6^(10 * -7) = 6^-70
C. 1/2^3 * 1/2^5...not this one...because when multiplying with the same base, u keep the base and add the exponents
D. 5^2 * 5^4...not this one...because when multiplying by the same base, u keep the base and add the exponents
so ur answer is B