B) 8x = 72 is your answer
You figure this out by using cross multiplication which is taking the denominator (The bottom number) from the first fraction, multiplying it by the numerator (The top number) of the second fraction, and then setting it equal to the numerator of the first fraction, and multiplying it to the denominator of the second fraction. This will basically look like an X, or a cross, thus it's name cross multiplication.
So for this problem it'll be 8x = 12(6) For the answer though they simply solved 12(6) and changing it to 72, thus your answer B) 8x = 72
Hope this helps!