To find A ∩ B, we need to find the elements that are common to both A and B.
A = {y : multiple of 3} contains all the numbers that are multiples of 3 between 1 and 20. So, A = {3, 6, 9, 12, 15, 18}.
B = {z : odd numbers} contains all the odd numbers between 1 and 20. So, B = {1, 3, 5, 7, 9, 11, 13, 15, 17, 19}.
To find A ∩ B, we need to find the common elements of A and B. The common elements of A and B are 3, 9, and 15. Therefore, A ∩ B = {3, 9, 15}.
So, option C - {3, 9, 15} is the correct answer.