Explanation:
The occurrence of endemic species in South America and Australia is primarily due to continental separation. Here's a detailed explanation of why this is the correct answer:

Continental Separation:
- Continental separation refers to the physical separation of two landmasses by the process of plate tectonics. In the case of South America and Australia, both continents were once part of the supercontinent Gondwana, which began to break apart around 180 million years ago.
- As the continents drifted apart, they created barriers to the movement of species between them. This isolation allowed for the development of unique ecosystems and the evolution of distinct species in each region.
- Over millions of years, the isolated ecosystems in South America and Australia became home to a wide range of endemic species that are found nowhere else in the world. These species have adapted to the unique environmental conditions of their respective regions and have evolved in isolation from species in other parts of the world.

Other Options:
Let's briefly discuss why the other options are not the correct answers:

a) These species have been extinct from other regions:
- This option suggests that the endemic species in South America and Australia were once present in other regions but are now extinct there.
- However, there is no evidence to support this claim. Endemic species are typically unique to a specific region and have not been found in other areas.

c) There is no terrestrial route to these places:
- This option suggests that the lack of a terrestrial route to South America and Australia is responsible for the occurrence of endemic species.
- While it is true that both continents are geographically isolated, this alone does not explain the presence of endemic species. Other factors, such as the length of isolation and the availability of suitable habitats, are also important.

d) Retrogressive evolution:
- Retrogressive evolution refers to the loss or reduction of complex traits in a species over time.
- There is no evidence to suggest that the endemic species in South America and Australia have undergone retrogressive evolution. In fact, many of these species are highly adapted to their environments and have evolved unique characteristics that allow them to thrive in their respective habitats.

Overall, the occurrence of endemic species in South America and Australia is primarily due to continental separation, which has led to the development of isolated ecosystems and the evolution of distinct species in each region.