Final answer:
The Agency for Healthcare Research and Quality (AHRQ) actively maintains the National Guideline Clearinghouse, focusing on the improvement of healthcare services quality.
Step-by-step explanation:
The organization that actively maintains the National Guideline Clearinghouse is the Agency for Healthcare Research and Quality (AHRQ). The AHRQ is a part of the U.S. Department of Health & Human Services and is responsible for developing enhancements to healthcare by ensuring that research findings are understood and implemented. While organizations such as the National Institutes of Health (NIH) and the Centers for Disease Control and Prevention (CDC) function in different capacities, such as medical research and public health protection respectively, it is the AHRQ that specifically focuses on quality improvement in healthcare services, which includes maintaining the National Guideline Clearinghouse.