Each of the risk factors listed is indeed a major concern when thinking about the overall health of an individual adolescent. Any of those situations, conditions, factors, etc. can have life long consequences. All of them have ties to overall mental health as well, and unfortunately, each of them could end up with early death from malnutrition, starvation, suicide, or overdose. None of those risk factors should be taken lightly.
Of the risk factors listed, substance abuse could pose the greatest risk to the adolescent. Substance abuse is dangerous for a number of reasons. One reason is that many of the substances that are abused are highly addicting. This means that getting off of the drug is incredibly difficult. The body craves the chemical, but the body also develops a tolerance to the chemical, so more of the substance is needed to achieve the same effect (or a more powerful substance is needed). Substance abuse tends to escalate, and it is the user that is escalating the risk which is different than something like child abuse.
An adolescent that is using these substances, is putting mind and body altering chemicals into their system, and many of those chemicals are things that can cause life-long brain chemistry changes. This means that even if the adolescent quits the drug, the effects are there for the rest of the person's life. This can be said about abuse and other mental health issues, but often psychiatric therapy can help; however, brain chemistry changes from substance abuse can be permanent. In general, I think substance abuse poses to the greatest threat to adolescents because of the wide variety of substances available and the potentially permanent and long lasting effects.