In Bohr's model of an atom, which letters are used to represent the orbits?

What are the various letters used by Bohr to represent electron shells in an atom?

In Bohr's model of the atom, the orbits or shells are represented by the letters and numbers. Identify the correct statement.

Why was a change in the Bohr Model of atom required? Due to which important development(s), concept of movement of an electron in an orbit was replaced by the concept of probability of finding electron in an orbital? What is the name given to the changed model of atom?

In Bohr model of the hydrogen atom, the lowest orbit corresponds to