38a) To find out if the material affects the time taken for the ice to melt

This is based on the changed and measured variables of her experiment. She could be trying to investigate which is the poorest conductor of heat, too. 

c) Using C-E-R (claim-evidence-reason)

C: Material Z

E: The ice melted the fastest

R: Z is the best conductor of heat, so the food can (gain heat quickly) and cook fastest in the pot