How to remove one 'document' by 'ID' using the Official C# Driver for MongoDB?

That's the way you do it. I'm sure you know this, but if you want to put it on one line you could combine it so you don't need to define a query variable:

collection.Remove(Query.EQ("_id", a.Id));

If the [id] is string, you must use ObjectId instance explicitly.

var query = Query.EQ("_id", ObjectId.Parse(id));

The Simplest Way

Remove a document from a collection for C# MongoDB Driver (v2.0 or later)-

collection.DeleteOne(a => a.Id==id);

Or-

await collection.DeleteOneAsync(a => a.Id==id);