Answer:
The particles of frosting absorbed energy from their surroundings and changed from the solid to the liquid state.
Step-by-step explanation:
On a sunny day, the energy of the particles in the surrounding is higher than the energy of the frosting tray of cupcakes.
As a result of the exchange of energy between the tray of cupcakes and the surrounding, the tray of cupcakes absorbs energy from the surrounding. When this occurs, the firm frosting is now converted from solid to liquid which is runny and wet to touch, hence the answer above.