What Does It Mean to Say, "Stock Prices Follow a Random

What does it mean to say, "Stock prices follow a random walk?"

A) Stock prices move haphazardly and, thus, are unpredictable.
B) Differing transaction costs result in different signals for investors, so excess returns to investors are random.
C) The stock market is both imperfect and inefficient.
D) Stock prices move in response to new information, but new information enters the market randomly.