Which function is used to convert a string to uppercase in Python?

The function used to convert a string to uppercase in Python is upper(). This built-in method is called on a string object and returns a new string where all the lowercase letters in the original string have been converted to their uppercase counterparts. It's a straightforward and commonly used function when you want to ensure that the text representation is in uppercase form, which can be particularly useful for standardizing input, formatting for display, or making comparisons case-insensitive.

For example, using the upper() function like this:

text = "hello"

uppercase_text = text.upper()

will result in uppercase_text being "HELLO". This is the correct and most efficient way to achieve the conversion to uppercase in Python.

The other options listed do not exist as standard string methods in Python, making them unsuitable for converting strings to uppercase.