Final answer:
The correct sentence that demonstrates the correct separation of elements in a series is option A: walking to school, we saw a dog, a raccoon, and a squirrel, and Ada, my sister, also saw a turtle.
Step-by-step explanation:
The correct sentence that demonstrates the correct separation of elements in a series is option A: walking to school, we saw a dog, a raccoon, and a squirrel, and Ada, my sister, also saw a turtle. In this option, commas are used to separate the different items in the series (dog, raccoon, and squirrel), and a comma is used before the coordinating conjunction 'and' to separate the last item (squirrel) from the rest of the series. Additionally, a comma is used to separate the appositive phrase 'Ada, my sister' from the rest of the sentence.