The simple subject of this sentence is B. discovery.
Which is just an interrogative pronoun, it cannot be a subject. Important is an adjective, which cannot be a subject. You is an objective pronoun.
If this is for We Wear the Mask By Paul, then the answer is D
C is the answer of the questions
Answer: Cosmonauts are people trained and certified by the Russian Space Agency to work in space. Astronauts are people trained and certified by NASA, ESA, CSA, or JAXA to work in space.
Explanation: