Which of the Following Statements Provides the Most Serious Obstacle

Which of the following statements provides the most serious obstacle to the use of introspection as a source of scientific evidence?

A) When facts are provided by introspection, we have no way to assess the facts themselves, independent of the reporter's particular perspective on them.
B) Introspection requires an alert, verbally expressive investigator; otherwise, the evidence provided by introspection will be of poor quality.
C) Introspection provides evidence about some mental events but cannot provide evidence about unconscious processes or ideas.
D) The process of reporting on one's own mental events can take a lot of time and can slow down the processes under investigation.