What is the purpose of the 'frooble' function defined as follows: def frooble(l): '''(list of str) -> int (precondition: l has at least one element.)'''
(a) It calculates the length of the string list 'l'.
(b) It returns the index of the longest string in 'l'.
(c) It sums the lengths of all strings in 'l'.
(d) It returns the length of the first string in 'l'.