In the following sentence completion question, choose the most appropriate preposition to complete the sentence: After retirement Surti lived _______ Australia.

Introduction / Context:
Preposition questions test whether a learner knows which prepositions naturally collocate with particular nouns, especially place names. The sentence After retirement Surti lived _______ Australia refers to living in a country. English uses in with large areas such as countries and big cities to indicate residence. The other options in the question either misuse common prepositions of place or combine them in unnatural ways, making them incorrect for standard written English.

Given Data / Assumptions:

• Sentence: After retirement Surti lived _______ Australia.
• Options: in, at, over in, over at.
• We assume the context is permanent or long term residence in a country.
• We follow standard grammatical conventions for prepositions of place.

Concept / Approach:
The general rule for geographical locations is that we use in with countries, large cities, and enclosed spaces; we use at for specific points such as addresses, events, or small locations; and on for surfaces. With the name of a country such as Australia, the idiomatic and grammatical choice is in. Expressions like over in or over at can appear in informal speech to add emphasis to distance, but they are not used in the simple, neutral pattern of a sentence like this in exam English. Therefore, in is the correct preposition to complete the sentence.

Step-by-Step Solution:
Step 1: Identify the type of place in the sentence. Australia is a country, a large geographical area. Step 2: Recall the rule that for countries we normally say live in a country, for example live in India or live in Canada. Step 3: Insert in: After retirement Surti lived in Australia. This sounds natural and correct. Step 4: Test at: After retirement Surti lived at Australia. This is ungrammatical because at is used for specific addresses or locations, not whole countries. Step 5: Test over in: After retirement Surti lived over in Australia. Although possible in very informal conversation, it adds an unnecessary over and is not the simple, standard form expected in exams. Step 6: Test over at: After retirement Surti lived over at Australia. This is not idiomatic in standard English and sounds incorrect. Step 7: Conclude that in is the only appropriate choice.

Verification / Alternative check:
Consider similar correct sentences: She lives in Germany, They settled in New Zealand, My uncle lives in Japan. In each case, in is used with a country. The only time at might be used is with a small local landmark or exact address, such as at 12 Park Street or at the station. Over in and over at may occasionally appear in spoken English to hint at distance, as in He lives over in Canada, but exam standards favour the simple and grammatically clear form, which is in. This confirms in as the correct preposition for this question.

Why Other Options Are Wrong:
At is wrong because one does not normally say live at plus a country name. It would be appropriate with a small, precise location. Over in is wrong in the neutral, simple style expected in competitive exams, even though it may be heard in informal speech. Over at is also wrong because it is used for specific locations, not for countries, and sounds awkward in combination with Australia. None of these alternatives fits standard written English usage for describing residence in a country.

Common Pitfalls:
Some students may be tempted by over in because they have heard it in conversation, but exam questions usually expect clean, standard forms without extra informal particles. Another pitfall is confusion between in and at, especially for city names, but remembering that both countries and cities typically take in for residence is helpful. Practising with many examples like live in Mumbai, live in France, and live in London helps to fix the pattern in memory.

Final Answer:
The correct preposition is in, so the sentence should read: After retirement Surti lived in Australia.