Luckily, math is just my subject! ;)
(If you want the answer, it's at the bottom, if you want the explanation, it's here)
Starting this, you can see that the only quote on quote (route) here is on 2k^4 in which, its 4... clearly.... anyways, seeing that, means you can cancel out A. and B. for having ^5. So your left with 6k^4 and 8k^4. This isn't addition, it clearly has a multiplication symbol, therefore, it can not be C. instantly ruling it out leaving the answer as D. 8k^4. Alrighty then, hope you have a wonderful day! :D