Option b).
Because view is responsible for all the UI logic within MVC pattern.
Controller does not care whether the user's action originated from drop-down menu or some button. For controller it only matters, that based on said action, it will alter the state of model layer.
And model layer, while possibly being the source of available options, does not know how the options where presented. Look at it this way:
Which part of triad you would have to alter to transform the drop-down into a radio-group?