B. Water; thirst
Explanation:
Water is very vital for survival, even in animals such as dogs. Water is around 90% of blood, so as you can imagine it is very important to consume water regularly to not get dehydrated.
The feeling of thirst is a very important internal stimuli which makes us want to drink water, as our body is fueled on water along with numerous other things. If the water content in Spot's blood gets low, the body will set off a signal for him to get thirsty, which then leads to him getting up to find water.
This is just a way the body continues to work as great as it does, even in dogs! Internal stimuli signals like thirst, hunger, and the feeling of being tired keep our body in check.
