Concept:The correct structure is “so + adjective + that” to show a result.Explanation:The sentence needs a word that pairs with “that” to express consequence.“Too” means more than enough, but it does not pair with “that” in this pattern.“Very” and “much” cannot be followed by “that” in this construction.Only “so” fits correctly: “so simple that...” means the test was simple to such a degree that everyone expected to pass.Thus, the correct option is C.Answer:C. so