Final answer:
To clean a difficult stain, it would be best for a porter to use bleach, which is a powerful cleaning agent capable of disinfection and tackling tough stains, though it must be used with caution.
Step-by-step explanation:
To clean a stain that is difficult to remove, it would be BEST for a porter to use bleach. Bleach is a powerful cleaning agent that can tackle tough stains and is effective against a wide range of microorganisms. It is especially useful in situations where disinfection is also desired. While other substances like soap and water, vinegar, and ammonia can be used for various cleaning tasks, they might not be as effective as bleach on difficult stains.
It should be noted that bleach must be used with caution, as it is a harsh chemical that can damage certain surfaces and fabrics. Additionally, when using bleach, it is important to ensure proper ventilation and to follow all safety instructions on the product's label.