D. (4x^2 - x + 2)
reasoning:
coefficients (the # in front of the variable) gets divided by the 3, and the exponents essentially just get subtracted.
12x^4 divided by 3x^2 (divide 12 by 3 and subtract the exponents to reach 4x^2)
same for -3x^2 divided by 3x^2 (since these terms are the almost same, you would just end up with -1)
6x^2 divided by 3x^2 (divide 6 by 3 which equals two, and subtract the exponents. that leave you with just 2 since 2-2 = 0 for the exponents)
place it together back in standard form
4x^2 - x + 2