Explanation:

- Option 1: Memorable day

- A "red letter day" is a day that is special or memorable, often marked for celebration due to a significant event or achievement.

- Correct answer.

- Option 2: Rose day

- Refers typically to a day associated with gifting roses, often celebrated during love-centric weeks like Valentine’s.

- Not related to the meaning of "red letter day."

- Option 3: Holiday

- While holidays can be memorable, not all holidays are referred to as "red letter days."

- More specific to memorable or significant occasions.

- Option 4: Birthday

- Refers to the anniversary of someone's birth, which can be a memorable occasion for individuals.

- However, it is not the typical meaning of "red letter day."