Concept:Use "such a" before a singular countable noun that is preceded by an adjective.Explanation:The phrase "such a" means "of this kind" or "so great."It is used with the pattern: such a + adjective + singular noun.Here, "wonderful conclusion" is a singular countable noun with an adjective.So the correct expression is "such a wonderful conclusion".The other options are grammatically incorrect.Answer:D. such a