Which built-in function is used to calculate the length of a sequence like a list or string?

The correct answer is Option A: len().
The len() function in Python is used to calculate the length of a sequence, such as a list or a string. It returns the number of items in the sequence. For example, len([1, 2, 3]) will return 3, as there are three elements in the list. Similarly, len("hello") will return 5, as there are five characters in the string "hello".