The function 'fetchrow_hashref()' returns reference to hash of row values keyed by . . . . . . . .

This question asks about the fetchrow_hashref() function in MySQL. This function is used to get a row of data from a result set and store it as a hash, which is like a special kind of list where each item has a unique key.

The question wants to know what these keys are. The answer is column names.

Here's a simple explanation:
Imagine a table in your database with columns named "Name", "Age", and "City". When you use fetchrow_hashref(), it returns a hash where:
* The keys are "Name", "Age", and "City" (the column names).
* The values are the actual data from that row (like "John", "30", "New York").

So, the correct answer is Option B: column name.