The answer is
.
Let’s find the ratio of the potential differences across capacitor C2 for the two cases.
In the first case, we have two identical capacitors connected to a battery with emf V. The potential difference across each capacitor is V/2.
In the second case, a dielectric slab with dielectric constant k fills the gap of capacitor C2. The potential difference across capacitor C2 is now given by:

where C1 and C2 are the capacitances of the two capacitors.
The ratio of the potential differences across capacitor C2 for the two cases is:

Simplifying the expression, we get:

Therefore, the ratio of the potential differences across capacitor C2 for the two cases is 2 / (2 + (k - 1) * C1/C2).
The answer is
.