Unlike secondary succession, primary succession
a. occurs in an area where existing communities still exist.
b. proceeds at a much faster rate.
c. leads to an increased number of different species over time.
d. has the challenge of occurring in areas where no community exists at all

In this problem, the most probable and the most likely answer to be chosen among the presented options would be D. has the challenge of occurring in areas where no community exists at all.

This would be one of the many differences of the secondary succession and the primary succession. While secondary successions usually happen when there is a catastrophe that hit the place which caused its previous dwellers to go and transfer to another one, the primary successions happens when there aren't even dwellers to begin with.