The Automated Testing Framework (ATF) is the best tool for quickly validating critical processes after a ServiceNow instance upgrade. Here's why:

Automated Execution: ATF allows you to create automated tests that can be run quickly and repeatedly after the upgrade. This significantly reduces the time required for testing compared to manual methods.  

Comprehensive Coverage: You can create automated tests for various processes, workflows, UI actions, and business rules, ensuring comprehensive validation of critical functionality.  

Regression Testing: ATF is particularly valuable for regression testing, ensuring that the upgrade hasn't introduced any unexpected issues or broken existing functionality.  

Why not the other options?

A. Test Management 2.0: While Test Management 2.0 provides a framework for managing tests, it doesn't inherently shorten the testing time itself. It can be used with ATF to organize and track automated tests.  

C. Manual testing scripts: Manual testing is time-consuming and prone to errors, especially for repetitive tasks involved in upgrade validation.  

D. System health dashboard: This dashboard provides an overview of system performance and health, but it doesn't directly validate specific processes or workflows.