Final answer:
The query argument passed to the pymongo function "find_one()" is a Python dictionary that serves as the query filter for MongoDB documents.
Step-by-step explanation:
When using the pymongo library's find_one() function, the query argument type expected is a dictionary. In Python, a dictionary is a collection of key-value pairs, enclosed by curly braces, with each key being separated from its value by a colon. For instance, if you wanted to find a document within a MongoDB collection where the field name is equal to 'Alice', you would pass the following query argument to find_one():
{'name': 'Alice'}
This dictionary serves as the query filter, specifying the criteria that the database document must match to be returned by the find_one() method. Hence, understanding Python dictionaries is essential for formulating correct queries when interfacing with MongoDB through the pymongo driver.