What tool is primarily designed for cleaning and consists of absorbent material fastened to a handle?

The tool primarily designed for cleaning and consisting of absorbent material fastened to a handle is the swab. Swabs are specifically crafted for tasks that require precision cleaning or sampling, such as cleaning surfaces in a laboratory, collecting samples for analysis, or even for medical applications. The absorbent tip of the swab allows it to effectively pick up fluids or debris, making it an ideal choice for applications requiring thorough cleaning or contamination prevention.

In contrast, the other options do not serve the primary purpose of cleaning. Test tubes are containers used for holding, mixing, or heating liquids. Test tube racks are designed to hold the test tubes in an upright position but do not have cleaning properties. Transfer pipets are used for transferring small amounts of liquid but do not have absorbent materials attached to a handle for cleaning purposes. Thus, the swab is the most fitting answer for the described characteristics.