Let's break down the sentence and find the best fit for the blank:

Analyzing the Context:
- "Although his association with Olivia Wharton has brought his ideas into the mainstream..." This indicates a positive influence; his ideas are now more widely accepted.
- "...some of Dr. Stephen Garner's more $\qquad$ theories are still not accepted by all of his peers." The word in the blank needs to describe theories that aren't accepted, even though some of his ideas have gained mainstream acceptance. This suggests the theories in question are likely to be unusual, disputed, or likely to cause disagreement.

Let's evaluate the options:
- A. controversial: Giving rise or likely to give rise to public disagreement. This fits perfectly. Even if some ideas are mainstream, controversial ones might still be rejected.
- B. demanding: Requiring much skill or effort. While theories can be demanding to understand, this doesn't explain why they might be rejected.
- C. famous: Known to many people. This is the opposite of what's needed. If they were famous, they would likely be accepted.
- D. divisive: Tending to cause disagreement or hostility between people. This also fits well, similar to controversial. Divisive theories are likely to be rejected by some.
- E. endemic: Regularly found among particular people or in a certain area. This describes prevalence, not acceptance.
- F. conducive: Making a certain situation or outcome likely or possible. This is the opposite of what's needed. If they were conducive, they would be more likely to be accepted.

Both controversial and divisive describe theories that would be likely to be rejected by some peers, even if other ideas have become mainstream.