Which of the following transitions will require the most thermal energy?
a)1 g of ice at –15 °c to 1 g of ice at –5 °c
b)1 g of ice at –5 °c to 1 g of water at 5 °c
c)1 g of water at 5 °c to 1 g of water at 15 °c
d)1 g of water at 15 °c to 1 g of water at 25 °c

Here, 1 g of ice at –5 °C to 1 g of water at 5 °C will require the most thermal energy 'cause it will require some extra energy (latent heat = 80 Cal/gm for this case) for changing the phase from 0 C ice to 0 C water.

In short, Your Answer would be Option B

Hope this helps!