Which of the following electron configurations of neutral atoms represent excited states?

( a) [xe]6s24f1
( b) 2s2
( c) 1s22s22p63s23p63d2
( d) [ar]4s23d3
(e ) [kr]5s14d5

i think it's c. if you look search electronic symbol on google and then click on wiki it shows you a

5 structural isomers

the isomers are:

n-exane :   ch3ch2ch2ch2ch2ch3

2-methylpentane :   ch3ch(ch3)ch2ch2ch3

3-methylpentane :   ch3ch2ch(ch3)ch2ch3

2,3-dimethylbutane :   (ch3)2chch(ch3)2

2,2-dimethylbutane :   (ch3)3cch2ch3