The correct option is C preferable to
In the underlined word, 'preferable' is a comparative by itself, hence it doesn't need 'more' before it. Also, 'preferable' is followed by the preposition 'to', and not than. Hence option C is correct, and D is incorrect. Option A is incorrect because 'more' is unnecessary in the statement. Option B is incorrect because the adjective is followed by 'to', not 'than'.