Explanation:

- Option:3, Once upon a time

- This phrase is correct. It sets the scene for a past event.

- Option:4, a beautiful princess

- This part is grammatically sound. It introduces the subject of the sentence.

- Option:1, has fallen in love

- This segment is incorrect. "Has fallen" is in the present perfect tense, which doesn't match the past narrative introduced by "Once upon a time." It should be "fell in love."

- Option:2, with a poor farmer

- This part of the sentence is correct. It describes who the princess fell in love with.

- Correct Answer: Option:1, has fallen in love