When is a Plugin Class Contsructor fired on a WP site

I’m trying to build a cache for data captured by a plugin I am writing and I have some questions on when are plugin constructors fired when word press uses them?

I see three options:

1. When the plugin is activated on the site.
2. When the web page server is accessed by a client.
3. When the client accesses the plugin specific features.

Thanks ahead of time, and details are appreciated.