To find one item from mongo collection, I am trying to apply filter and to the collection. But there is a compilation error as below.

This code is taken from official mongodb
docs
var filter = Builders<BsonDocument>.Filter.Eq("_id", id);
var result = _collection.Find(filter);
Best How To :
Generic Type of Builder
should be the same as for collection's generic type. In your case collection should have type BsonDocument.
var _collection = database..GetCollection<BsonDocument>("name");
var filter = Builders<BsonDocument>.Filter.Eq("_id", id);
var result = _collection.Find(filter);