After playing a difficult soccer match that wore his body out, jon slept for an extra 4 hours. jon's extra sleep is best explained by the theory that states that the function of sleep is to

Restorative theory

Explanation:

The restorative theory is the theory that restores our tissue and prepares us for the next day. It works when the g lymphatic symptoms flush from the brain during the sleep night. It releases adenosine chemical that is responsible for increasing the level of sleepiness during a wakeful condition.  

Thus here in the above statement Jon brain release adenosine that is responsible for sleepiness during the wakeful situation.  

the answer is d.index