Final answer:
The closest appropriate size ET tube for intubating a two-year-old toddler from the given options is 4.0 mm, especially if using a cuffed tube.
Step-by-step explanation:
When intubating a two-year-old toddler, the appropriate size for an endotracheal tube (ET tube) is typically 4.5 mm in internal diameter for uncuffed tubes, and 4.0 mm if a cuffed ET tube is used. Since size 4.5 mm is not an option listed, and a 5.0 mm might be too large, the closest appropriate size from the options would be a 4.0 mm cuffed ET tube. It's crucial to ensure the correct size is used to prevent damage to the child's airway and to maintain adequate ventilation.