The letters 'a', 'e', 'h', 'o', 'r', and 't' have already been guessed. The current word contains 'a', 'r', 't', and the remaining letters should be guessed to find the secret word. The letters 'c', 'd', 'f', 'g', 'i', 'j', 'k', 'l', 'm', 'n', 'p', 'q', 's', 'u', 'v', 'w', 'x', 'y', 'z' are still available. Given this, 's' is a common letter that might fit well, especially as there seems to be a chance for a plural or a related word.