Sovereign' vs 'autonomous'?

[Rag Ram]

English Language Tests, Intermediate level

ESL/EFL Test #151 "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
(c) autonomous
(d) sovereign

English Language Tests, Intermediate level

ESL/EFL Test #151 "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: incorrect
Australia is a(n) autonomous country and therefore has the right to choose whether or not tourists are granted visas.
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'Sovereign' is nothing but 'autonomous'

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

[Mister Micawber]

.
Autonomous in geopolitical terms refers to a part of a country that has a measure of self-government.
.
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Native English teacher at Mister Micawber's

[Saneta]

a) independent
(c) autonomous
(d) sovereign

What are differences between them above?

Thank you very much