The correct option is C The wise are not always the most skillful.
The given sentence is in the superlative degree of comparison, the superlative phrase here being 'the most skillful'. When an adjective describes a group of people, but is not followed by the noun in a sentence, it means that the adjective has taken the place of the noun hence the definite article 'the' should precede the adjective 'wise'. The article is missing in the given sentence, it needs to be added, and hence option A is incorrect. In option B 'the' precedes the adjective 'wise' and hence B is correct. Option C is incorrect because 'the' has been omitted from the sentence before 'wise' as well as 'skillful'. Superlative adjectives are always preceded by 'the' because it refers to only one such person or group. In option D, 'the' and 'most' have been omitted before 'wise' and 'skillful', since 'the' is the necessary article to use before 'wise' and skillful being in the superlative degree, the sentence is incomplete without 'most'. Hence option D is incorrect.