Answer
Potential energy is increased.
Work Step by Step
Since energy stored is $\frac{1}{2}C \Delta V^2$, and if $C$ increases when dielectric is placed between plates of capacitor with $V$ constant, then energy stored is increased.
You can help us out by revising, improving and updating this answer.
Update this answerAfter you claim an answer you’ll have 24 hours to send in a draft. An editor will review the submission and either publish your submission or provide feedback.