option B is answer. L2 may not be regular, This can be see by following example-
let L1 = Σ∗ and L2 = {anbn∣n>0} . Now, (L1 ⋃ L2) is Σ∗ which is regular but L2 is not regular.
B is the Answer
check this for detailed explanation https://gateoverflow.in/3876/given-that-a-language-la-l1-u-l2
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