If you set an option's pricing tier to required, at least one of that tier will need to be booked by every customer booking that option. If you set more than one of an option's pricing tiers to required, at least one of any of the required tiers will need to be booked.
To flag a pricing tier as required, first navigate to the option's edit page.
Under Pricing Tiers, you can see all current pricing tiers that can be booked through that option.
To make a tier required, click the required/optional toggle. The orange checkmark and "required" or "req" label mark it as required. The grey X and "optional" or "opt" label mark it as optional.
Click Save Changes when you finish editing pricing tiers.