The correct answers are A. Intransigent and F. Dogged.

Explanation
The sentence establishes a contrast between the subject's early career and his later years. The keyword "flexible" describes his past, while the word "surprisingly" signals that his later behavior was the opposite of flexible. We are looking for words that describe a stubborn, unyielding attachment to ideas.
- Intransigent means refusing to agree or compromise; uncompromising and inflexible.
- Dogged means having or showing tenacity and grim persistence; stubbornly unyielding.

Both words create a cohesive meaning: despite being open-minded in the 1920s, he became stubbornly fixed in his ways later in life.

Why the other options don't work:
- Vacillating (B): This means wavering or indecisive. This is closer to being "flexible" and therefore does not provide the necessary contrast.
- Sectarian (C): While "sects" is mentioned in the sentence, "sectarian" describes the nature of the ideas, not the manner of his adherence. It doesn't contrast with being a "flexible thinker."
- Confused (D): One can be confused and flexible, or confused and stubborn. It doesn't specifically target the "inflexibility" required by the context.
- Frantic (E): This implies panic or wild energy, which doesn't necessarily mean one is refusing to change their mind.