Multi Select Drop Down List

I'm trying to think of a way where I have a select form with multiple "Categories" in the array, how can when I select a Certain category it prefills another Multi select with the associated options for that category.

Forexample Category List Below

1. Chassis Maintenance
2. Floor Renovation
3. Paint

Subcategories: When Above Selected category it has its own sub select list

Chassis Maintenance

1. Change Oil
2. List item
3. Lube Chassis
4. Change Air filter

and so on

So per Selected Category it has its own sub category

Any Idea?