There is regular trade between two parties through letter of credit but they want that it may not be required to open a letter of credit again and again. What type of LC is suitable for them?

A revolving letter of credit is a special letter of credit type which is structured in a way so that it revolves either in value or in time covering multiple shipments over a long period of time under single letter of credit. As, in the given case, they don't want to open an LC again and again, so revolving LC is suitable for them.