Which entity is tasked with improving aviation safety through investigating accidents?

The National Transportation Safety Board (NTSB) is the correct choice because it has the primary responsibility for investigating transportation accidents, including those in the aviation sector. This independent federal agency conducts thorough investigations into aviation incidents and accidents, analyzing the circumstances, contributing factors, and potential safety improvements. The NTSB also makes safety recommendations to various stakeholders to help prevent future accidents.

In contrast, the Federal Aviation Administration (FAA) primarily oversees the regulation of civil aviation and the safety of air traffic operations, as well as the certification of aircraft and personnel. While it plays a vital role in promoting safety, it does not investigate accidents in the same capacity as the NTSB.

The Department of Transportation (DOT) encompasses a broader scope of transportation oversight, which includes the FAA and NTSB, but it does not directly conduct accident investigations.

The Aircraft Owners and Pilots Association (AOPA) is an organization that advocates for general aviation and represents pilot interests, but it does not have a role in accident investigation or improving safety through such investigations. Thus, the NTSB's specialized focus on accident investigation and safety recommendations makes it the correct answer.