What It Does
This formula will return a random selection from a given list with each item in the list having an equal probability of being selected.
How It Works
ROWS(List) will return the number of rows in the List. In our example there are 4 rows in the list.
RANDBETWEEN(1,4) will return a random integer number between 1 and 4 where each number has an equal probability of being selected. This number is then used as the row index to return an item in the list.
INDEX(List,N) will return the item in row N of the List. In our example N will be a random number between 1 and 4.