4: A group of people were told that red M&M's caused cancer.  Later they
  were told that they had received incorrect information and that red
  M&M's were absolutely safe.  According to research on belief
  perseverance, the people

D: bought stock in the company that makes M&M's

    Incorrect.
Belief perseverance is where older beliefs remain intact, in spite of new information that presents convincing evidence the old belief is inaccurate or irrational.  In this situation, some newer information (e.g. buying stock in the company that makes M&M's, which may reflect confidence that M&M's don't cause cancer) has altered the original belief (e.g. M&M's cause cancer), contradicting research on belief perseverance.