Final answer:
A usage dictionary would be the most helpful reference to understand what Stanley means when he uses the word preen. Option c.
Step-by-step explanation:
The reference that would be most helpful in understanding what Stanley means when he uses the word preen is a usage dictionary.
A usage dictionary provides information on how words are used in a specific context or domain, such as literature or a particular profession. It goes beyond the ordinary speech definitions provided in a regular dictionary and gives insights into the specific nuances and connotations of words.
By consulting a usage dictionary, Stanley can gain a deeper understanding of how the word preen is used within a specific domain or field, which can help him accurately interpret its meaning in the given context.
So option c.