With these types of questions, it is critical that we refer to the way that the words in question are used in the passage rather than to their typical definition.
Lines 47-50 read: "Literature considered for the first time at the appellate level is not subject to live comment by practicing experts and cannot be tested in the CRUCIBLE of the adversarial system." Here, the word "crucible" is used to describe a larger process/system/instrument.
Lines 39-43 read: "And adverse parties can test the credibility and reliability of proffered literature by subjecting the expert witness to the greatest legal ENGINE ever invested for the discovery of truth - cross-examination." Here again, the word "engine" is used to describe the larger process/system/instrument.
Lines 59-62 read: "This criticism applies with full force to the use of outside-the-record texts and treatises, regardless of the MEDIUM in which they are found." Here, the word "medium" does not refer to a larger process/system/instrument but instead to a specific form/means.
Does that make sense? Please let us know if you have any other questions!