By elemination we can conclude that answer is C
But.
Although I understand that L5 is regular. But i dont inderstand how L4 is regular.
CSL is closed under concatenation and since
L1.L3*
CSL. REG*
CSL. REG
Since every regular language is also a CSL and its a strict subset. Therefore
CSL.(REG union $)
CSL. CSL
wiz. CSL only.
How L4 can be regular ?