The update() method in Python is used to add an element to a dictionary. It takes either another dictionary object or an iterable of key-value pairs and adds them to the dictionary. If the key is already present in the dictionary, update() will update the key with the new value. Option B correctly uses the update() method to add the new 'N9K-Spine-2': '1498323434' key-value pair to the switch_id_list dictionary.
References: For more information on modifying dictionaries in Python, the official Python documentation provides comprehensive guidelines and examples. It's a valuable resource for understanding the behavior of dictionary methods like update().