Which 'round' is used as a preposition?

Same words can be used as different parts of speech.
For example, 'round' as different parts of speech:

Preposition -- The earth revolves round the sun. (চতুর্দিকে)
Adjective -- A square peg in a round hole. (গোলাকার)
Noun -- The evening was a round of pleasures. (চক্র)
Adverb -- He came round to their belief. (ফিরে আসা) 
Verb -- We shall round the cape in safety. (ঘোরা)