I have a form in which there is combobox of job type. When i select the "Sales representative " from combobox a list of buttons from A to Z will appear by using AJAX like"
A B C D E F G ....... Z.
When we click on particular button for eg .if I click on C the "Dealers" Combobox will generate which has the values starting with "C" letter,if click A the combobox will load the values starting with "A" letter. Next When we select the value of dealer from combobox the next "Dealer branch" combobox will generate and load values of dealer branch related to that dealer. Next when we select the value from dealer from dealer branch the next combobox of "Sales Manager" will generate and it load the values of sales manager related to that dealer branch.
I can't solve this problem can you plz help me?
You can get the majority of the functionality you described using the Hierarchical Select Module. Read that page and it's documentation here. You may end up needing to create a custom module and use Forms API which is no easy feat and has a learning curve.
Forms api has a states functionality that makes AJAX much simpler. Read this for more info about the Forms API. Read this for more info about states.