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Explanation:

In option 1, “firing a round of tear gas shells” means ‘a single discharge by one firearm’ and the word ‘round’ is correctly used.
The phrase, ‘round the corner’ in option 2 means, ‘nearby, a short distance away’.
In option 3, the usage is correct. “Merry-go-around” is a ‘carrousel in amusement parks, carnivals, etc. A merry-go-round is a revolving, circular platform with wooden horses or other animals, benches, etc., on which people may sit or ride, usually to the accompaniment of mechanical or recorded music.’
In option 4, the usage “on a hospital round” is correct. This is used to signify that the doctor is surveying the wards checking on patients.
“Come round” in the fifth option means ‘to change one's opinion, decision, especially to agree with another’s.’ A better option would have been to come around to admitting it meaning to change one’s position or opinion.
Hence, the correct answer is option 5.