The correct answer is D. Space exploration is extremely expensive, but the knowledge gained is worth the price.
Explanation:
Even though the sentence "Space exploration would cost a ton of money, but I would probably learn a lot of new things, so it’s probably worth it" is grammatically correct it can be clearer and more concise as the use of "space exploration would cost" and " I would probably learn" is too ambiguous, in this sections of the sentence the writer does not deny or affirm anything and thus meaning is not completely clear.
However, by replacing "Space exploration would cost a ton of money" by "space exploration is extremely expensive" and "But I would probably learn a lot of new things, so it's probably worth it" the meaning is clearer as it focuses on the main point of each part, then, the main idea of the first section is how space exploration is and how despite this space exploration is worth. Thus, the revision proposed in D. is the best as it makes the meaning of the sentence clearer and creates and more concise sentence by avoiding extra words that are not necessary or might confuse the reader.
