This riddle says a woman has four keys on a ring, but only one opens her front door. Without trying any of the keys, how does she know which one it is?
The answer is simple: she recognizes her own front-door key because she has used it before.
Keys often differ in shape, size, color, markings, or position on a key ring. Someone who uses the same key regularly does not need to test every key each time.
The riddle feels tricky because it makes you imagine that the woman has never seen the keys before. But the question never says that. They are her keys, and she can know the correct one from familiarity.
Sometimes the simplest explanation is the intended answer.
