Exponential growth is a type of population growth in which the number of individuals in a population increases rapidly over time. It is characterized by a constant growth rate, with each generation having more offspring than the previous one. However, there are certain limitations and characteristics associated with exponential growth that make options C and D incorrect.

Exponential Growth in Nature:
a) They do not exist in nature for a long period of time.

Exponential growth does occur in nature for a period of time, especially when populations are not limited by resources or environmental constraints. In the absence of limiting factors, such as competition for resources or predation, populations can exhibit exponential growth. For example, when a new species is introduced to an area with abundant resources and no natural predators, its population can grow exponentially until it reaches the carrying capacity of the environment.

Shape of the Growth Curve:
b) It produces a J-shaped curve.

Exponential growth does produce a J-shaped curve when plotted on a graph. Initially, the population size remains relatively small, and as the growth rate remains constant, the population increases gradually. However, as the population size increases, the rate of growth also increases, leading to a rapid rise in population size. This results in a characteristic J-shaped curve, where the population size increases dramatically over time.

Sustainability of Exponential Growth:
c) It is a sustainable model of population growth.

Exponential growth is not a sustainable model of population growth in the long term. While it can occur in the absence of limiting factors, such conditions are rarely found in nature. Eventually, populations reach their carrying capacity, which is the maximum number of individuals an environment can support. When this happens, resources become limited, and the growth rate slows down, leading to a more stable population size. Thus, exponential growth is not sustainable in the long term.

Intraspecific Competition:
d) It can cause intraspecific competition.

Exponential growth can indeed cause intraspecific competition, which is the competition for resources among individuals of the same species. As the population size increases rapidly, individuals must compete for limited resources such as food, water, and shelter. This competition can lead to increased mortality rates, reduced reproductive success, and other negative effects on the population.

In conclusion, exponential growth can occur in nature for a period of time, it produces a J-shaped curve, it can cause intraspecific competition, but it is not a sustainable model of population growth in the long term.