A 1 mF, a 2 mF, and a 3 mF capacitor are connected in series, the combination being connected across a 9 volt battery. Which capacitor has the greatest voltage?

Answer 1

C = charge per unit voltage
so
V = Q/C

Like the problem below this one, since they are in series, only One charge, call it Q, comes out of the battery and lands on the top of the first capacitor. That draws a -Q from the next one which leaves +Q on the top etc.
In the end, each capacitor has charge Q
since V = Q/C
the smallest C has the biggest V

Answer 2

1 mF