Explanation
The sentence states: "Gauging the genetic diversity of deep-sea microbial communities presents formidable obstacles; only after amassing samples across decades can researchers attain the resolution needed to $\_\_\_\_$ shifts that carry any evolutionary significance." The key clue is "resolution" - the ability to distinguish fine details. The purpose of decades of data collection is to be able to perceive, detect, or identify subtle changes. The blank must mean detect, discern, or determine.

The correct pair must be tight synonyms that both describe the act of perceiving or identifying something that is difficult to observe.
- A. engender - To cause, produce, bring about. Researchers do not collect data to cause shifts; they collect data to detect them. Incorrect.
- B. discriminate - To perceive the distinguishing features; to recognize a difference; to discern. "Attain the resolution needed to discriminate shifts" perfectly captures the ability to distinguish meaningful changes from noise. Correct.
- C. abide - To tolerate, endure, put up with. Researchers do not collect data to tolerate shifts. Incorrect.
- D. contemplate - To think about, consider, envisage. "Contemplate" is too passive and speculative. The goal is to detect, not to ponder. Incorrect.
- E. ascertain - To find out with certainty; to determine; to discover. "Attain the resolution needed to ascertain shifts" produces a sentence virtually identical in meaning to the one with "discriminate." Both describe identifying changes. Correct.
- F. endure - To suffer through, withstand, tolerate. Like "abide," this is completely wrong for the context. Incorrect.