In Amazon Lex, slots are used to extract specific pieces of information from user utterances. In this case, "tomorrow" is a date-related value that needs to be captured as input for processing the appointment booking.

• Intent: Represents the overall goal or action the user wants to perform (e.g., "BookAppointment")
• Slot: Captures specific data points like dates, names, locations, etc.
• Lambda Function: Used for additional processing logic
• Alias: Used for version management of bots

The correct answer is B) Slot because slots are specifically designed to extract and process variable inputs like dates, names, and other parameters from user messages.