Activating a Button...

If you are using html5 you can include this JavaScript or insert it in the head of the html document.

There is a function call ed myClick where you can show or enable the button. Just put the smartshape button name in the quotes:

var interfaceObj, eventEmitterObj;

window.addEventListener( 'moduleReadyEvent', function ( e )
{
interfaceObj = e.Data;
eventEmitterObj = interfaceObj.getEventEmitter();
initializeEventListeners();
});

function initializeEventListeners()
{
if ( interfaceObj )
{
     if ( eventEmitterObj )
  {
         eventEmitterObj.addEventListener( 'CPAPI_SLIDEENTER', function ( e )
   {    
    if ( window.cpInQuizScope )
    {    
     res = cp.model.data.project_main.slides.split( ',' );
     cSlide = res[ window.cpInfoCurrentSlide - 1 ];    
     thisSlide = cp.model.data[ cSlide ];
 
     for ( var i = 0; i < thisSlide.si.length; i++ )
     {     
      if ( cp.model.data[ thisSlide.si[ i ].n ].hasOwnProperty( 'qdi' ) )
      {
       var temp = cp.model.data[ thisSlide.si[ i ].n ].qdi.toString();      
       document.getElementById( temp ).addEventListener("click", myClick);
      }
     }
    }

   
    function myClick( e )
    {
     cp.show( 'smartshape_btn' );
     cp.enable ( 'smartshape_btn' );    
    }
     
            });  
  }
}
}

Each action, and disabling/enabling a shape button is an action, has to be triggered by an event.

On question slides you only have the On Enter event, and the Success/Last Attempt events. The last events occur after the two-step process for Submitting, not after a selection of an answer.  If you explained the goal of that button, maybe another approach could be possible? I explained some use cases for shape buttons on question/score slide in this article:

http://blog.lilybiri.com/want-a-button-on-question-slide-in-captivate

If you want to see a complete list with available events out of the box in Captivate:

http://blog.lilybiri.com/events-and-advanced-actions

There is no run-time event available from selection of an answer option that you could use to trigger an Advanced Action that SHOWs or ENABLEs a button on the Quiz Slide. 

You'd need some kind of custom programming or a specialized widget to pull this off, but those quiz answer captions do not have Object Names that can easily be referenced by a widget to trigger an event.