What is a tool that helps us identify and classify different things?

The correct choice is indeed a key. In scientific classification and identification, a key is a systematic tool that allows users to differentiate between various species or items based on specific characteristics. This tool typically takes the form of a dichotomous key, where users are presented with paired statements or questions about traits, leading them step-by-step to the correct identification of the organism or object in question.

For instance, a key might be used in biology to classify plants or animals based on observable features such as leaf shape, flower color, or size. This process is essential in fields such as taxonomy, ecology, and environmental science, where accurate identification is crucial for research, conservation, and management practices.

While a compass, map, and guide offer valuable functions in navigation and location, they do not serve the specific purpose of identifying and classifying items based on distinct criteria, which is the focal point of the question. Thus, a key is uniquely suited to the task of classification.