Explanation:

Correct answer is (b). The bold part of the statement has error since proper parts of speech has not been used in this part of the sentence. If we want to modify an adjective, we should use an adverb to do the same rather than having another adjective preceding it. Unusual complicated is not the correct usage since both are adjectives and they cannot be used like this. An adverb should have been placed before complicated to modify the same. Unusually complicated would have been the correct usage in this context.
The correct sentence would be: The Supreme Court decided to refer the unusually complicated issue to a larger bench of the apex court so that proper justice is meted out to the citizens of the country.
This makes option B the correct choice among the given options.