Final answer:
The suffix indicating a specialist in a field of study is '-ist', transforming fields like psychology and biology into professions like psychologist and biologist.
Step-by-step explanation:
The suffix that denotes a specialist in a field of study is -ist. This suffix transforms a field of study into a noun that describes an expert or professional within that field.
For example, a psychologist is an expert in the field of psychology, the scientific study of the mind and behavior. Similarly, a biologist is a specialist in biology, and a linguist is an expert in linguistics.