Sovereign' vs 'autonomous'?

English Language Tests, Intermediate level

ESL/EFL Test #151 [color=blue]“Traveling Down Under: How can I immigrate to Australia?”, question 6

Australia is a(n) country and therefore has the right to choose whether or not tourists are granted visas.

(a) independent
(b) governing
© autonomous
(d) sovereign

English Language Tests, Intermediate level

ESL/EFL Test #151 [color=blue]“Traveling Down Under: How can I immigrate to Australia?”, answer 6

Australia is a(n) sovereign country and therefore has the right to choose whether or not tourists are granted visas.

Correct answer: (d) sovereign

Your answer was: [color=red]incorrect
Australia is a(n) autonomous country and therefore has the right to choose whether or not tourists are granted visas.
[size=200]_________________________[/size]

‘Sovereign’ is nothing but ‘autonomous’

then why can’t we use autonomous in this sentence ?

.
Autonomous in geopolitical terms refers to a part of a country that has a measure of self-government.
.

a) independent
© autonomous
(d) sovereign

What are differences between them above?

Thank you very much