Choose the most suitable one-word substitution for the following expression.

A person who specialises in the scientific study of plants.

The correct option is Botanist.

Let us break down the meaning of the given expression and compare it with all the options to find the most suitable one-word substitution:

Given Expression: "A person who specialises in the scientific study of plants."

1. Botanist: A scientist who studies plant life, including their structure, growth, reproduction, metabolism, diseases, and chemical properties. This perfectly matches the given statement.

2. Geologist: An expert in or student of geology, which is the scientific study of the origin, history, and structure of the Earth (rocks, minerals, and landforms).

3. Agronomist: An expert in the science of soil management and crop production (agriculture).

4. Mycologist: A scientist who specializes in mycology, the study of fungi (such as mushrooms, molds, and yeasts).

Therefore, the word that best substitutes the given phrase is Botanist.