In Amazon Lex, slots are used to extract specific pieces of information from user utterances. When a user says "Book an appointment for tomorrow," the word "tomorrow" is a date/time value that needs to be captured as input for processing. Slots are designed to extract such parameter values from natural language input.

• Intent (Option A): Represents the overall goal or action the user wants to perform (e.g., "BookAppointment"), not the specific parameter extraction.
• Slot (Option B): Correct answer. Slots extract specific data values like dates, names, locations, etc., from user input.
• Lambda Function (Option C): Used for additional processing or validation, not for direct extraction of parameters from utterances.
• Alias (Option D): Typically refers to bot aliases for version management, not for parameter extraction.

Therefore, the correct answer is B) Slot.