Вместо (?) нужно вставить one или ones:
How old are my children? The younger (?) is four and the older (?) is seven. 2. The new mobiles are much lighter than the old (?) . 3. I don't mind what kind of car it is, I just want (?) that gets me there. 4. Which do you prefer; this (?) or that (?) ? 5. I need some new glasses. The (?) I have at the moment are broken. 6. I hope this holiday will be (?) to remember.