You have a custom Web Part.

You need to create a custom user interface for modifying the Web Part properties.

What should you do?

A.
Modify the [ToolBox] attribute of the custom Web Part.

B.
Create a new tool part for the custom Web Part.

C.
Create a new Web Part. Implement the IControlBuilderAccessor interface.

D.
Create a new Master Page. Implement the IControlBuilderAccessor interface.

Explanation:
MNEMONIC RULE: “Tool part for the Web Part”

What is a custom tool part?
The Custom tool part is part of the web part infrastructure, that helps us to create a custom user interface for the web part properties that goes beyond the capabilities of the default property pane.

When do we need a custom tool part?
Let’s say, If we need to create a web part property of type dropdown, we need to create a custom tool part. This is not supported out-of-box in the web part framework. I’ve the similar requirement of creating a custom web part property of type drop-down.

Create Custom Tool Parts for SharePoint Web Parts
https://msmvps.com/blogs/sundar_narasiman/archive/2009/09/02/create-custom-tool-parts-for-sharepoint-web-parts.aspx