Well, falls in old age can be a result of any of these things. Think about it...
A.) osteoporosis causes bones to become weak.
B.) Arthritis causes your joints to inflame .
C.) Poor nutrition could lead to dizziness and cause you to lose your balance.
D.) And obviously poor balance leads to falls.
So really, all of the above can cause falls in elderly people.
I guess I would go with D just because old people can be clumsy. Not really sure...
Hope this helped!