See folks I was also confused at first but now its cleared, lets see how
We all know why option d is correct since recursive lang is closed under complementation and hence L' is also Recursive lang therefore L' is RE lang also
BUT what if the language is regular lang. , if L is regular that means its CFL also but CFL's complementation property is not closed right? therefore L' is not CFL and hence not Rec. lang and hence not RE lang.
but you may argue that if L is regular and we first check if its L' is regular or not and since its regular therefore L' is CFL hence CSL and hence recursive and hence RE lang. but thats not the procedure first we need to go up till the position we can in the language itself till complementation property gets violated like we went up from regular to CFL and there complementation property got violated and hence we became sure ok this will not be our answer.
now you may again argue then why did we not go from recursive lang. to RE lang. ,its because in the question its mentioned L is RE lang as well as L' is RE lang.
Hope I could clear it a bit