Final answer:
Turner's safety-valve theory posits that western expansion served as a critical outlet for economic and social pressures in the East, providing opportunities for new beginnings and alleviating urban congestion.
Step-by-step explanation:
Frederick Jackson Turner's safety-valve theory implied that the migration to western territories provided a release for societal pressures in the East. With the availability of land in the West, individuals encountering economic hardships or social discontent could migrate, pursue new opportunities, and relieve the congested urban areas in the East. Turner believed that this westward movement served as a critical outlet, preventing the escalation of social tensions by offering an alternative to wage labor and scarce farmland, thus benefiting both those who moved and those who stayed behind.
Option D) All of the above, captures the essence of Turner's safety-valve theory which encompasses the potential prevention of social unrest, as an outlet for discontented individuals, and the opportunity to start anew for those facing hardships.