Which object must be related in order to create an opportunity record?

While Opportunities can originate from converted Leads, a Lead record itself is not directly related to an Opportunity during its creation; it's the Account and Contact created from the Lead that are subsequently related.

In Salesforce, an Opportunity represents a potential sale and must always be associated with an Account, which represents the company or organization you are doing business with.

An Opportunity can be related to multiple Contact Roles, but a Contact is not a mandatory direct parent object for Opportunity creation; the Account is the required parent.

Quotes are optional records that can be created from an Opportunity; an Opportunity does not need a Quote to exist or to be created.