Which of the following describes a qualitative analysis?

Qualitative analysis is primarily concerned with identifying the constituents of a substance rather than quantifying them. It aims to detect what kinds of chemical species are present in a sample, providing information about the types of substances without necessarily measuring their amounts. By focusing on the identification aspect, qualitative analysis allows chemists to ascertain the presence of particular ions, molecules, or compounds in a sample, which is essential in fields like forensic science, environmental monitoring, and clinical diagnostics.

The other options focus on quantitative aspects, such as counting molecules or measuring concentrations, which fall under quantitative analysis rather than qualitative. Thus, recognizing the role of qualitative analysis as a method for identifying substances aligns perfectly with the correct choice.