Explanation
The sentence describes Janna Levin defending "sweeping cosmological narratives" that "strike more cautious colleagues as utterly _______ ." Her rejoinder clarifies: "without such leaps into the empirical unknown, the field would stagnate into mere bookkeeping." The phrase "leaps into the empirical unknown" reveals that these narratives go beyond current evidence - they are speculative, lacking in data, or not yet supported by observation. The blank must mean lacking evidence, without foundation, or unsupported by proof.

The correct pair must be tight synonyms that both describe claims made without adequate factual support.
- A. unsubstantiated - Not supported or proven by evidence; lacking factual foundation. "Strike more cautious colleagues as utterly unsubstantiated" perfectly captures the criticism that the narratives lack empirical backing. Correct.
- B. hasty - Done too quickly; premature. While "premature" and "hasty" are close, they describe timing (too early), not evidentiary status (lacking proof). A claim can be hasty yet wellsupported; a claim can be unsubstantiated yet offered at the right time. The rejoinder emphasizes the empirical unknown, not the timing. Incorrect.
- C. contentious - Causing disagreement; controversial. While bold speculations are likely contentious, the specific criticism from "cautious colleagues" is about lack of evidence, not level of dispute. A claim can be contentious yet well-supported by data. Incorrect.
- D. demonstrable - Able to be proven or shown clearly. This is the opposite of what the blank requires. If the narratives were demonstrable, cautious colleagues would not object to them. Incorrect.
- E. falsifiable - Able to be proven false; testable. This is a positive trait in science - falsifiability is a criterion of scientific validity. Cautious colleagues would not criticize a claim for being falsifiable. Incorrect.
- F. groundless - Without foundation or basis in fact; baseless. "Strike more cautious colleagues as utterly groundless" produces a sentence virtually identical in meaning to the one with "unsubstantiated." Both describe claims lacking empirical support. Correct.