The correct option is D the most important city
The Superlative Degree denotes the existence of the highest degree of the quality. It is used when more than two things are compared.
The given sentence is in superlative form so the correct adjective here must be in superlative degree, i.e. 'the most important city'. Also the emphasising adjective 'the' should be used before 'most' and not before the noun 'city'.
Hence, OPtion A is the correct answer and Option C, Option B and Option D are incorrect.