these are great mock-ups! at first glance, i like 'a' the best because it gives the object prominence while still incorporating links to more info about the collection and tools for citing, sharing, and other ways to use the object. it also looks a little sleeker. i also like the way you handled the right side of the screen in 'd' with twitter and fb options near links to collections info so that similar items can be seen with minimal scrolling. it will be interesting to hear what users have to say about them.