Is the Salesforce.com/Force.com platform a "relational database"?

I think it falls under the object-relational umbrella. Sure it does relationships, but it does more than that, and with the abstraction of the API you never really get at the full relational structure of the underlying oracle DB.

Answer number four is good I accidentally marked it as a negative

Yes it is, the platform is built on top of an oracle relational database.

@Gary just because the Force.com platform is built ON TOP of a relational database doesn't mean that it technically is one. It may have some similar constructs to a relational database, but it has some unique characteristics that make it markedly different, and also lacks some distinctive characterstics of a relational database. If I was interviewing you and you told me that the Force.com platform is a relational database because it was built on an Oracle DB I would give you +.01 points for knowing some relatively useless trivia about the underlying architecture and -10 points for not understanding the unique characteristics and limitations of the platform.