According to a 2018 article written by Dr. Mark Stibich, suicide is the 2nd cause accounting for 18% of deaths among people age 15-24.
According to the CDC's 2007 statistics, homicide was the 2nd leading aide of death among people age 15-24.
According to the National Institute of Mental Health 2017 statistics, suicide was the second leading cause of death among individuals between the ages of 15 and 34.
According to the Population Reference Bureau (June 2016), suicides becsme the second cause of death among teenagers in the United States, surpassing homicide deaths.
I would say suicide is the answer, but it will depend on what year's statistics your class reference material is using.
