Final answer:
Neil and Michaela made an external, unstable attribution.
Step-by-step explanation:
Neil and Michaela have made an external, unstable attribution to explain why Vincent fell over.
When they commented, 'I didn't know that the floor was so slippery in this part of the hall!' they attributed the cause of Vincent's fall to an external factor, which is the slippery floor in that specific part of the hall. This attribution is considered external because it attributes the cause of the fall to something outside of Vincent's control.
Additionally, their attribution is considered unstable because it implies that the slipperiness of the floor in that specific part of the hall is not a regular or constant occurrence. It suggests that the slipperiness is a temporary condition, which may vary over time.