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

The correct option containing the error is then.

Step-by-step Explanation:
1. Identify the comparative expression: The sentence introduces a choice or preference using the expression "Rather then...".
2. Distinguish between "than" and "then":
- Than is a conjunction used to introduce the second element in a comparison or to express choice (e.g., "rather than").
- Then is an adverb used to indicate time, sequence, or consequence (e.g., "We went to the park, and then we went home").
3. Apply the rule: In this context, a comparison/preference is being made between going to a crowded city and spending the weekend in the countryside. Therefore, the prepositional comparative phrase must be "rather than" instead of "rather then".
4. Check the other options:
- crowded: Properly used as an adjective modifying the noun "city".
- tranquil: Properly used as an adjective meaning calm or peaceful, modifying "countryside retreat".
- retreat: Properly used as a noun meaning a place of seclusion or privacy.