Explanation:

The question asks for a feature to route cases from social channels within a "limited timeframe." This implies the need for efficient and potentially time-sensitive routing.

A. Use custom case assignment rules:
This is a fundamental and declarative Salesforce feature for routing cases. When a social post is converted into a case via Salesforce Social Customer Service, it becomes a standard case record. You can then configure case assignment rules to automatically assign these cases to specific queues or agents based on criteria such as:
Case Origin: Set to 'Social' or the specific social channel (e.g., 'Facebook', 'Twitter').
Priority: Route high-priority social cases to a dedicated queue.
Business Hours/Time: While assignment rules don't directly have time-of-day criteria, you can use them in conjunction with business hours (for escalation rules) or a Flow that updates a field based on time, which then triggers an assignment rule. This allows for routing social cases differently during or outside specific "limited timeframes" or business hours. This is a standard and efficient way to handle case routing.

B. Implement an Apex solution:
Apex is a custom code solution. While it can achieve any complex routing logic, it's generally considered a last resort due to higher development cost, maintenance, and the need for developer resources. For routing cases based on origin and time (even indirectly), declarative tools like assignment rules are usually sufficient and preferred.

C. Use a third-party app from AppExchange:
Many AppExchange apps offer advanced social media management and sophisticated routing features. While this could be a valid solution for very complex or highly specialized social routing needs (e.g., sentiment-based routing, real-time social listening integration beyond native capabilities), for the general requirement of routing cases "within a limited timeframe," standard custom case assignment rules are a native, cost-effective, and effective solution. The question does not specify complexity that mandates a third-party app.

Therefore, custom case assignment rules are the most appropriate and standard declarative solution for this requirement.

Reference:
This topic falls under the Interaction Channels and Case Management sections of the Salesforce Service Cloud Consultant exam.
Salesforce Help & Documentation - Case Assignment Rules: This documentation details how to set up and use assignment rules to automatically assign incoming cases based on specified criteria.
Trailhead - Automate Basic Case Management: This module covers how to use assignment rules, escalation rules, and other automation tools to manage cases efficiently, including those from social channels.