Explanation:
(3m-4)² + 8(3m-2) = (3m-4)(3m-4) + 24m - 16 =
= 9m² - 3m×4 - 4×3m + 16 + 24m - 16 =
= 9m² - 24m + 16 + 24m - 16 = 9m²
so, there is seemingly something missing in your original expression.
going from 9m², the most likely solution would be 9m/4 and therefore B.
but we cannot be sure, because again, I don't know what you left out in the beginning.