Rather then going to the crowded city, they decided to spend their weekend in a tranquil countryside retreat.

The correct option is then.

Step-by-step Explanation:
1. Analyze the given sentence: "Rather then going to the crowded city, they decided to spend their weekend in a tranquil countryside retreat."
2. Identify the grammatical error: The word "then" is used incorrectly in the phrase "Rather then."
3. Distinguish between "than" and "then":
- Than: Used as a conjunction for comparisons or to express preferences (e.g., "rather than").
- Then: Used as an adverb to indicate time, sequence, or logical consequence (e.g., "We went to the store, and then we went home").
4. Conclusion: Since the sentence is presenting a choice or preference between the crowded city and a tranquil countryside retreat, the comparative conjunction "than" should be used. Therefore, "then" is incorrect, and the corrected sentence should read: "Rather than going to the crowded city..."