Concept:The correct order for emphasis with a singular countable noun is "such + a + adjective + noun".
Explanation:The noun in the sentence is "conclusion", which is singular and countable.
It is preceded by the adjective "wonderful", so the article "a" must come after "such" and before the adjective.
Therefore, the correct phrase is "such a wonderful conclusion".
Option A is incorrect because "a such" reverses this required order.
Option B is incorrect because "that a" does not express the intended meaning and is ungrammatical here.
Option C is incorrect because "that such" cannot be used before a singular countable noun in this structure.
Only option D correctly uses "such a" to mean "of that type or quality".
Answer:D. such a