Question about infinite series

[badatcalc]

To anyone that can help me with this - You have to pick the FIRST correct reason. Work below (exception of 4 because I cannot figure it out), but in order to get the question right you must have all correct and I cannot figure it out. Any help is appreciated.

[Moderator's note: Moved from a technical forum and thus no template.]

 

[PeroK]

I'd double check 4 & 6.

 

[Delta2]

No4 is not that hard once you figure out that $6^{2n}=(6^2)^n=36^n$ and $(6^2+1)^n=37^n$.

No6 looks fine to me. (oh now I see it says the FIRST reason, might be A afterall).