When we read the question, this is a brief, pseudo-syllogistic argument, so we will use term matching.

Evidence Term	Matches?	Conclusion Term
Astrology uses scientific methods		Astrology is a true field of science

  The conclusion comes at the beginning of this argument; "Astrology is a true field of science" is the opinion that draws on the other facts for support. The improperly matched term is "true field of science." The most critical assumption is roughly, "A field that uses scientific methods is a true field of science." That statement or a variant of it, such as its contrapositive, is what we're expecting as the answer. Testing this prediction, (C) and (D) both have the right terms involved. Choice (D) matches our prediction. Choice (C) is the logical inversion of what we're looking for, so it is not correct. We can look for logical confirmation of the correct answer: We can prove choice (D) with the negation test. Say that employing scientific methods did not make that field of study is true field of science. In that case, the argument falls apart. Since the negation of (D) destroys the argument, (D) itself is assumed by the argument.

The correct answer is (D).