Feasibility on Script Based Validation with Enumerated Attribute Validation

It will be a lot simpler than this example - something like this will work. I have just thrown this together without checking the code for errors and you will need to substitute the correct values for things. Note that the Validator is fired just before the Save, so you need to check for the final states B and C, not the initial state A

dojo.provide("example.Validator");

dojo.require("com.ibm.team.workitem.api.common.Severity");
dojo.require("com.ibm.team.workitem.api.common.Status");
dojo.require("com.ibm.team.workitem.api.common.WorkItemAttributes");

(function() {
  const WorkItemAttributes = com.ibm.team.workitem.api.common.WorkItemAttributes;
  const Severity = com.ibm.team.workitem.api.common.Severity;
  const Status = com.ibm.team.workitem.api.common.Status;

  dojo.declare("example.Validator", null, {

    validate: function(attribute, workItem, configuration) {
      const wiState = workItem.getValue(WorkItemAttributes.STATE);
      const needReview = workItem.getValue('<the id or label of the Needs Review attribute>');

      if (wiState === '<the internal id of State B>' && needReview !== 'No') {
        return Status(Severity["ERROR"], 'Please use State C to transition this work item, as a review is required');
      } else if (wiState === '<the internal id of State C>' && needReview !== 'Yes') {
        return Status(Severity["ERROR"], 'Please use State B to transition this work item, as a review is not required');
      }
      return Status.OK_STATUS;
    }
  });
})();

 As far as I can tell, there is no public supported JavaScript API that allows to do this. In attribute customization, you can not change the state.

The only possible approach I would be aware of, is a follow-up action/participant for work item save. It could detect the attribute change and perform one or more workflow actions to enter the desired state. https://rsjazz.wordpress.com/2012/11/27/resolve-parent-if-all-children-are-resolved-participant/ is an example that shows some of the required code. 

As Ralph says, there's no public way to change the State but you can do it using unpublished calls.

However, one way that is officially supported would be to create a condition - this would then prevents a Save if the conditions were not met and would flag an error that you can customise. You could then check the State and the attribute and stop people from transitioning to the wrong state.

Not exactly an automatic transition, but would enforce your desired workflow