How can I allow customer to select multiple options?

If you want to allow customers to select more than one item in an option, then you can use on of the following option types.

Checkbox

The "Checkbox" option type supports multi selection:

Click on "Add value" to add a new option item:

You can add as many values as needed:

Multiple option values can then be selected individually:

When the product is added to the cart, the selected option values are saved comma-separated:  

Swatches

The "Swatches" option types can also be configured to allow multi selection:

In the Advanced tab, enable the "Swatch multi select" setting:

Multiple swatch items can then be selected individually:

Conditions

The value of an option that has multiple values is stored separated by comma, e.g:  toppings=pineapple,mushrooms

If you want to create a condition on this option to only show it if a specific value was selected, you have to make sure to use the contains operator instead of equals: