The answer is b. The most important thought the author wants readers to understand is that too little sleep can cause stress. The dependent clause, which is introduced by although, tells readers information the author considers less important, that too little sleep is an effect of stress (that is, people who are stressed may not sleep well). Notice that if you read the dependent clause aloud to someone and stopped there, the listener would expect you to add something more. The dependent clause does not make complete sense by itself.