Final answer:
The proper punctuation option is number 2: 'I saw a duck billed platypus at the city zoo.'
Step-by-step explanation:
The proper punctuation option is number 2: I saw a duck billed platypus at the city zoo. In this option, the words 'duck' and 'billed' are not connected with a hyphen, since they are not functioning as a compound adjective. Additionally, the words 'city' and 'zoo' are not connected with a hyphen, since they are not forming a compound noun. The other options either use hyphens incorrectly or omit hyphens where they are needed.