How to remove a Required Field based on other field value i have a form with two fields, FieldA (Select List) and FieldB (Select List, required field).

How to set the FieldB to optional when FieldA has certain value in order to pass the form validation?
Try using the #states attribute: a structured array describing the different JavaScript states that can be applied to the element when certain conditions are met. Please read the extensive description of this feature at drupal_process_states()