Final answer:
Thomas Malthus theorized that unchecked population growth would outstrip available resources, leading to famine, disease, and war, acting as 'positive checks' to reduce the population.
Step-by-step explanation:
Thomas Malthus theorized that if left unchecked, a population will exhaust its available resources, resulting in naturally occurring checks on the population, such as famine, disease, and war. These are referred to as Malthusian disasters. He believed that population growth was limited by the means of subsistence and that excess population would inevitably be reduced by these 'positive checks' which increase mortality rates.
Therefore, the correct answer is B: If left unchecked, a population will exhaust its available resources, resulting in naturally occurring checks on the population, such as famine, disease, and war.