Bill and Betty are still MARRIED. Therefore it is impossible for him to be part of a social unit with anyone else. Invite Betty. Ask her if its okay to invite Bill by himself. Do not invite the girlfriend of a married man to the same party as his wife.
I would agree with this if they were not separated and actively in the process of an amicable divorce, especially because Carol had nothing to do with the separation or divorce. I would feel completely differently if Carol contributed to the breakup or if either Bill or Betty had hard feelings about the divorce. From the information we have, it seems Bill and Betty are not bitter so I see no reason to make things awkward for people who like them both. Personally, I would want to be trusted to handle situations as an adult, which to me means accepting that the marriage is over (even though the papers are not final) and being cordial and friendly to my ex-spouse and his new SO. Presumably, Betty's kids are around Carol so I see no reason why she would not want to be friendly with her.
I disagree. No one really knows how Betty would feel if he say her husband with his girlfriend. She might be okay with it or it might hurt her feelings greatly. She might be okay with it one day but not the next. Divorce, even an amicable one is a highly emotionally fraught time.
Maybe I'm a traditionalist, but marriage actually means something to me. This is a family event. I wouldn't want Betty's kids to have to deal with dad's girlfriend before the divorce was final nor would I want to have to explain to my kids that the lady holding hands with Mr. Smith is his new girlfriend even though the Smith's aren't divorced yet. Either you are married or you aren't. If you are still married you are absolutely not part of any other social unit.
I'm sure the marriage also meant something to Betty and Bill. I would invite Carol with Bill if Betty said it was okay. There doesn't seem to be any point in pretending that Betty and Bill are still in a relationship.
But that wasn't one of the four choices. None of the choice reflect that Betty and the hostess are lifelong good friends and Bill is a longtime spouse of the friend. The primary relationship
is with Betty, not Bill. I can think of no circumstance where I would go out of my way to befriend the new girlfriend of my BFF's exhusband.
In fact, how on earth would the hosts even know how committed Bill is to Carol if they've never seen them together? They have barely seen Bill since the divorce and Bill and Al seem to have no independent relationship
I would not suggest pretending they are still in a happy relationship
(ie sending and invitation to Betty addressed to "Bill and Betty". But inviting the girlfriend could be very very hurtful, even if the divorce is amicable, it is often very painful to both parties when a marriage ends, even if it is for the best.