The question could be looking for the key words: ” the holder expands MORE than the ball, hence the ball can be removed”. Your child’s answer only states the holder will expand, noting in fact both holder & ball will expand, though at different extent.
that’s kinda stupid isn’t it? considering it was specified that the metal holder was heated directly, why would we be expected to explain heat gain in comparison as if heat was applied equally to both?
(b) Metal is a good conductor of heat. When the metal holder is immersed in the hot water, the metal holder will gain heat faster than the ball and expand more than the ball. Hence, the metal holder will become bigger than the ball and they can be separated.