The correct answer is: A. Vertical
Under the vertical method of filling letters, letters are kept standing upright in a pigeonhole. This method is often used in mail sorting offices, as it allows for quick and easy access to the letters.
The horizontal method of filling letters is less common, but it can be used in situations where there is limited space. In this method, letters are laid flat in a pigeonhole.
The box method of filling letters is also less common, but it can be used in situations where there are a large number of letters to be sorted. In this method, letters are placed in a box, which is then sorted by hand.
The pigeon-hole method of filling letters is the oldest and most traditional method. In this method, letters are placed in a pigeonhole, which is a small compartment or box that is designed to hold a single letter.
I hope this helps! Let me know if you have any other questions.