2c is the current option. 2b and 2d are subsets of 2c.
2b (and 2a) is not a subset. There is a crucial difference between a metacoin and a user definied currency (as explained earlier): a metacoin is not issued (by any entity) and it is trusted by everyone accepting/trusting the protocol itself.
In terms of implementation, it is a subset.
Nope. In 2c) there is no coin/asset (apart from IXC) with a special status, i.e., there are
more things in 2b (and 2a) than in 2c. If you really want to see this as a "subset" thing, you can argue that 2c) is a subset of 2b) (or 2a) (with an additional freedom that any asset can be used for bets).