In some programming languages, an identifier is permitted to be a letter followed by any number of letters or digits. If L and D denotes the set of letters and digit respectively. Which of the following expression defines an identifier?

Automata theory (also known as Theory Of Computation) is a theoretical branch of Computer Science and Mathematics, which mainly deals with the logic of computation with respect to simple machines, referred to as automata. Automata* enables scientists to understand how machines compute the functions and solve problems.
Theory of Computation is very important in compiler design as it helps in writing efficient algorithms, which help make efficient compiler construcion.