(a) *iter++; is legal. The expression moves iter to point to the next element and returns the value of the original element.
(b) (*iter)++ is not legal. The expression means increasing value of the element, but the value is a string and string does not have ++ operator.
(c) *iter.empty() is not legal. Because iter is an iterator and has no member named empty.
(d) iter->empty() is legal. The expression means check if the string pointed by iter is empty.
(e) ++*iter is not legal. The expression means increasing value of the element, but the value is a string and string does not have ++ operator.
(f) iter++->empty() is legal. The expression means move iter to point to the next element and check if the original string is empty.