What is the correct way to write the underlined portion of sentence 7, reproduced here: "You may find sanitation workers working in the following places: city parks, food or beverage plants, restaurants, water treatment plants, office buildings, factories, warehouses, and industrial plants."

The colon is there to start the list of the variety of places one might find a sanitation worker. It is a lengthy series of items, so a colon would be an appropriate way to start that list. The sentence needs something to make the reader pause before the list (try reading it with no punctuation after "in the following places" and you'll run out of breath before you get to "industrial plants"), but a semicolon won't work because it's not two independent clauses coming together. A dash won't work because what follows isn't an interruption and isn't being added for emphasis.