Final answer:
A defined medium, also known as a synthetic medium, is a chemically defined culture medium used for growing bacteria. It can be selective, differential, or both.
Step-by-step explanation:
A defined medium, also known as a synthetic medium, refers to a type of culture medium used for growing bacteria in the laboratory.
It is chemically defined, meaning that the complete composition of the medium is known, including the precise amounts of each component.
A defined medium can be selective, differential, or both.
Therefore, the correct answer is option C:
a selective medium and a chemically defined medium.