To solve this question, the user needs to have knowledge of grammar and vocabulary. Specifically, the user needs to understand the meaning of the given sentence and how to correctly use modifiers to describe the level of difficulty of the test.

The correct answer is "so".

Explanation:

The word "so" is a modifier that is used to describe the level of difficulty of the test. It is the appropriate word to use because it indicates the degree of difficulty of the test and is used to express a result or consequence. It can be used to indicate the degree of the adjective "difficult" in the sentence.

Option A, "such", is incorrect because it is used to describe a specific type of noun, not a degree of an adjective.

Option B, "a", is incorrect because it is an article that is not used to modify adjectives.

Option C, "as", is incorrect because it is used to compare two things that are similar in some way. It is not appropriate to use it in the sentence above since there is no comparison being made.

Therefore, the correct answer is "so".

The Answer is: D.