Answer:
C. Because it was raining heavily, Monika arrived at her doctor's appointment fifteen minutes late.
Explanation:
If we want to express a causal relation, or that one event is the cause and the other its consequence, we need to use causal or consequential dependent clauses.
Leaving this sentence unchanged or revising it to the answer D, the reader might accnowledge these two events (raining and Monica's tardiness) as two independent, unrelated events.
Answer B. suggests that Monica rained heavily which is, of course, completely illogical and is an example of a misplaced modifier.
Only answer C. provides correlation between these two events. Connector "because" suggests that the rain was the cause of Monica being late
