<?php
if(in_array('In Stock', get_field('stock'))){
}else if(in_array('Forthcoming', get_field('stock'))){
}else if{in_array('Out of Stock', get_field('stock')){
}else{
No selected
}
?>
This should work then according to http://www.advancedcustomfields.com/resources/field-types/checkbox/
which I assume works the same as radiobuttons..