Final answer:
Lynn Margulis faced scientific skepticism and resistance to new ideas when she advanced the endosymbiotic theory. Over time, as evidence accumulated, the scientific community shifted from skepticism to wide acceptance of the theory.
Step-by-step explanation:
In answering the question about the obstacles Lynn Margulis faced in advancing the endosymbiotic theory, the correct choice is b) Scientific skepticism and resistance to new ideas. When Margulis published her ideas in 1967, the scientific community largely adhered to the gradualist theory of evolution. Margulis's theory, which suggested that key organelles in eukaryotic cells originated from a symbiotic union of different species of prokaryotes, was met with skepticism because it challenged the existing paradigm.
Furthermore, the high acceptance of evolutionary theory among biologists and the lack of peer-reviewed scientific literature refuting it underscores that resistance to Margulis's idea was not due to a general dismissal of evolution, but rather a resistance to a novel hypothesis within the evolutionary framework. Eventually, as microbiological evidence supported Margulis's hypothesis, the scientific community began to accept the theory as a fundamental explanation of the origin of eukaryotic cells.
The claim that scientists discard theories due to contradictory observations (answer d from the related question) can be related to Margulis's theory as well, which endured scrutiny until supportive evidence was widely recognized.