i)There is ample food and space for the population depicted by curve (a). When the resources are unlimited, the curves is exponential. There are limiting food and space for the population depicted by curve b). When the resources are limiting, are the curve becomes sigmoid.
ii) In the absence of predators, curve (a) would appropriately depict the prey population.
iii) The dotted line represents the carrying capacity of the environment. The carrying capacity represents the size of population that the environment can hold by providing necessary resources. when a population reaches this line its population size is stabilised by various environmental factors.