Which of the following is a classic category of study in botany?

Hydrology is indeed a classic category of study in botany because it focuses on the relationship between water and plant life. This field examines how plants interact with water, including aspects such as water availability, absorption, and its role in ecosystems. Understanding hydrology is crucial for studying plant distribution, growth patterns, and their adaptations to various water conditions.

The other options listed do not pertain directly to the study of botany. Hygiene relates to health and cleanliness, hyperthermia deals with body temperature regulation in living organisms, and chickpeas, while a plant, represent a specific subject of study rather than a broader category in botany. Thus, hydrology is recognized as a significant area within the broader scope of botanical studies.