Step 1: Analyzing the options.
The sentence needs to be arranged to describe the cause of the water crisis in the region.
- (A) starts by setting the context: "in the region," which places the location of the issue.
- (D) follows by identifying the source of the issue: "the pollutants from the factory."
- (C) connects to the consequence: "thus causing a water crisis."
- (B) explains how the pollutants spread: "had percolated into the underground water."
Step 2: Conclusion.
The correct order is: "In the region, the pollutants from the factory had percolated into the underground water, thus causing a water crisis."
Thus, the correct answer is 1. (A), (D), (C), (B).
Final Answer:
\[
\boxed{\text{The correct answer is 1. (A), (D), (C), (B).}}
\]