there is an unreal number i, i²=-1
so i³=i²i=-i, and i^4=(i²)²=1
i^27=

B and C are also correct. They will both end up being -i.
an easy easy is the divide the exponent by 4, if there is no remainder, the answer is 1. if the remainder is 1, the answer is i. if the remainder is 2, the answer is -1. if the remainder is 3, the answer is -i
I hope this helps.