What does the 'len()' function in Python return?

The 'len()' function in Python is designed to return the number of elements in a data structure. This function is commonly used with various types of collections, such as strings, lists, tuples, sets, and dictionaries. For example, when applied to a string, it returns the number of characters in that string. When used with a list, it tells how many items are present in that list.

Using 'len()' is a straightforward way to understand the size of containers in terms of the number of items they hold, making it essential for tasks involving data manipulation, indexing, and validation of input.

The other options do not accurately represent the functionality of the 'len()' function. Lengths of an integer or the size of a variable in bytes are not returned by 'len()'; instead, they would require different approaches or functions. Hence, the focus on the number of elements in a data structure is the key feature that validates the correctness of the chosen answer.