A parallel-plate air capacitor has a plate area of 100cm³ and separation 5mm. A potential difference of 300V is established between its plates by a battery. After disconnecting a battery, the space between the plates is filled by ebonite (K = 2.6). The new potential difference between plates of the capacitor is

115V 

After Battery is disconnected the charge remains constant.

$\Rightarrow Q' = Q \Rightarrow C'V = CV' $

$\Rightarrow KCV = CV' , V'= \frac{V}{K} = \frac{300}{2.6} = 115V$