Generally you might have a look at current "realtime" web frameworks like MeteorJS which tackle exactly this problem.
Meteor in specific works more or less like your example D with subscriptions on certain data and deltas being sent out after changes only to the affected clients. Their protocol used is called DDP which additionally sends the deltas not as overhead prone HTML but raw data.
If websockets are not available fallbacks like long polling or server sent events can be used.
If you plan to implement it yourself i hope these sources are some kind of inspiration how this problem has been approached. As already stated the specific use case is important
