Answer:
The correct way to rewrite this sentence is "my class’ garden"
Explanation:
The original sentence "The garden of my class" uses a form of possessive with the preposition "of", then from the given options the correct one is "my class’ garden" since it respects the original message by using the same singular nouns and modifying it by changing of for and ', the other options are either talking about a plural noun or they are omitting the possessive grammar.
