What is a key point? Like a key point of a novel. Would it be a main idea??

Which word correctly completes the sentence? both tracks are visible in the snow. a. deers' b. deer's c. deers 2.which word correctly completes the sentence? a huge diamond decorated the crown. a. kings' b. king's c. kings 3.which word correctly completes the sentence? at the old library, many of the covers were worn or damaged. a. books' b. books c. book's 4.which word correctly completes the sentence? in the traffic jam, cars moved at a pace. a. turtles' b. turtle's c. turtles plz asap