You got it wrong, the arms are meant to complement the headsculpts colouring, not the body.
AC Cloud headsculpts are a lighter tone compared to RE versions, hence the difference in arms colours options. The 1st class body colour tone is for AC Cloud version but comes with jointed arms. Think of the Arms set as an additional add on pack instead.
If you are buying the 1st class body to use the extra AC Cloud headsculpt from the deluxe version, you should get Arms A
If you are using the original RE Cloud Headsculpt, its Arms B
If it's for RE Zack, it's Arms C.
The extra body is just an additional option for collectors (mainly AC Cloud deluxe collectors who wanted to use the extra headsculpt).