Answer:
a. natural law
Explanation:
"Causal connection" is related to the connection of two events together. This means that one event causes the occurrence of the other.
The situation above is linked by "natural law." For example, the "downpour of rain," which is natural, caused the "oversaturated hillside" to slide. Both of these are naturally-occurring, without the interference of any force or other unnatural events.
The fire is also a natural event which "destroyed the root system." The "massive mudslide" is also a natural occurrence which caused "the damage in many houses."
So, this explains the answer.