What should be included in the update document to add an element to the items array field in the sales collection?

The correct approach for adding an element to an array in a MongoDB document is to use the $push operator. This operator is specifically designed to append a specified value to an existing array field within a document. When employing $push, the value you wish to add can be a single element or, when using aggregate expressions, multiple elements.

In this context, the syntax $push: { items: [{ "name": "tablet", "price": 200 }] } correctly specifies that a new object representing the item (with its name and price) should be added to the items array in the specified document within the sales collection.

The other options presented do not align with MongoDB's syntax or functionality for updating documents. For example, there is no $add operator for updating array fields; $add is generally used in the context of mathematical operations. Similarly, $insert and $append are not valid MongoDB operators. The correct operator, $push, is essential for correctly manipulating documents to add new elements to an array, making option A the right choice.