Which built-in Python function is used to convert a string to lowercase?

The built-in Python function used to convert a string to lowercase is str.lower(). This method is called on a string object and returns a new string where all the uppercase letters have been converted to lowercase. It is a crucial method in string manipulation, especially when you need to normalize text data for comparisons or storage, as case sensitivity can affect how strings are processed and compared.

Other options like str.to_lower(), str.downcase(), and str.lowercase() do not exist in Python's string methods. Python’s string class provides a straightforward and consistent way to handle various string transformations, and lower() is the specific method designed for converting string characters to their lowercase counterparts. Understanding how and when to use string methods like lower() is essential for effective text manipulation in Python.