Final answer:
Drug use is known to increase the risk of schizophrenia, especially when combined with genetic vulnerability and environmental stresses, with cannabis use in young people being particularly highlighted as a significant risk factor.
Step-by-step explanation:
The question asks which of the following increases the risk of schizophrenia: being an only child, college stress, drug use, or cigarette smoking in adolescence. Studies, such as those by Tienari and colleagues, support the notion that drug use can increase the risk of schizophrenia, especially when combined with genetic vulnerability and environmental stresses. Furthermore, cannabis use in young people is particularly highlighted as a risk factor for schizophrenia, as suggested by Casadio, Fernandes, Murray, & Di Forti (2011). Therefore, the answer is C) drug use. Smoking cigarettes in adolescence, while detrimental to health, is not directly linked to schizophrenia with the same weight of evidence as drug use.