Final answer:
In 'The Pilgrim's Progress' by John Bunyan, the slippery ground warned of by the four mistresses is 'Slime', representing moral dangers on a spiritual journey.
Step-by-step explanation:
The slippery ground that the four mistresses warn Christian of in John Bunyan's The Pilgrim's Progress is D) Slime. This allegorical reference is used to indicate a dangerous path or situation in one's spiritual journey that could cause a person to slip into sin or error. It suggests a treacherous obstacle that must be navigated with great care to avoid moral failure.