Answer:
Positive
Step-by-step explanation:
For the most effective hot pack, the temperature change should be positive.
Hot packs are generally utilized for their heat-producing abilities. Hence, the reactions leading to the activation of hot packs are usually exothermic, that is, heat energy generating. In other words, positive or increased temperatures must be generated when hot packs are activated.
This is unlike cold packs which are utilized for the cold-generating abilities. Reactions leading to their activations must be endothermic, that is, heat-absorbing.