Explanation:
The sentence sets up a transformation: the soul goes from [blank] theological abstraction → an entity that can be physically searched for "in the dark chambers of the human cranium." The core contrast is non-physical/immaterial vs. physically locatable.
- A. abstract and E. intangible are near-synonyms here - both mean "without physical form, existing only as an idea." Either word slots in to produce essentially the same sentence: the soul moves from being a purely conceptual/immaterial thing to something radiography could hunt for in physical space. This paired-synonym structure is exactly what SE requires - swap one for the other and the sentence's meaning doesn't change.

Why the others fail:
- C. abstruse - means "difficult to understand" (an intellectual/complexity quality), not "non-physical." It doesn't have a matching synonym among the other choices, and it doesn't set up the physical/non-physical contrast the sentence needs.
- D. ineffable - means "too extreme to be put into words." This actually undercuts the sentence, which says the soul was extensively discussed "in treatises and sermons" an ineffable thing resists being talked about at all. No synonym partner either.
- B. manifest and F. demonstrable - these mean roughly the opposite of what's needed (already visible/provable), so they contradict the "before" state, and using either would reverse the sentence's logic.