Explanation

The sentence explains the blank immediately after the dash:

"...the assumption that its engineering ranks share an unquestioning allegiance to a single inherited doctrine..."

This is essentially the definition of orthodoxy—strict adherence to established or traditional beliefs or doctrines.

The contrast is:

Old reputation: ideological orthodoxy (everyone follows the same inherited doctrine).
New evidence: internal surveys show a striking breadth of perspectives, contradicting that reputation.

A. uniformity | Plausible, since everyone supposedly thinks alike, but it doesn't capture the idea of adherence to an established doctrine as precisely.

B. insularity | Means being isolated or closed off from outside influences, not necessarily sharing one doctrine.

D. opacity | Means lack of transparency.

E. fractiousness | Means conflict or divisiveness, which is the opposite of the described assumption.

Answer: C.