Select the option that can be used as a one-word substitute for the given group of words.
The doctor who deals with the female reproductive system.

The correct answer is Gynaecologist.

Explanation:

A one-word substitute is a single word that replaces a longer phrase or sentence to express its meaning concisely.

Let's analyze the given options to understand their meanings:

1. Gynaecologist: A doctor who specializes in the health and diseases of the female reproductive system. (This matches the given group of words).

2. Ophthalmologist: A medical doctor who specializes in eye and vision care.

3. Orthodontist: A dental specialist who treats misaligned teeth and jaws.

4. Cardiologist: A doctor who specializes in diagnosing and treating diseases of the heart and blood vessels.

Therefore, the appropriate one-word substitute for "The doctor who deals with the female reproductive system" is Gynaecologist.