Check out local storage
construct.net/en/make-games/manuals/construct-3/plugin-reference/local-storage
As I´d do it:
1. Save all your options into a dictionary when setting them (e.g. volume: 80, fullscreen: true)
2. Then whenever (e.g. user clicks save options button or something like that) save it to local storage as json (dictionary.asJSON)
3. Then on game start you load it from local storage and into the dictionary
4. Last check all keys in the dictionary and reapply the settings (set volume to 80, if fullscreen is true -> browser request fullscreen)
That said, I´m not 100% sure that fullscreen can be saved and loaded like this, afaik the request fullscreen action has to be inside a user input (on touch/click)