Turing Recognizable:
Let's check it:
If L is CSL : It is turing recognizable as well as turing decidable
If L is RE : It is turing recognizable but may not be turing decidable.
Complement of CSL is CSL, but complement of RE may or may NOT RE.
(a) L and ̅L must be decidable. - CSL satisfies but RE not
(b) L must be decidable but ̅ L need not be. - Neither CSL nor RE satisfies
(c) Either L is decidable or ̅ L is not Turing recognizable. CSL not satisfies but RE satisfies
(d) None of above.
Answer should be C