Which of the following sentences is correctly punctuated?

Choice C is the correct answer. In this sentence, the use of the possessive form 'student's' correctly indicates that the best friend belongs to the working student. The apostrophe is placed before the 's' to show singular possession. The other options contain errors in punctuation. Choice A should be 'Several cars' bumpers' to indicate the bumpers belonging to multiple cars. Choice B should be 'Tom's and Anne's house' to show possession by both individuals. Choice D should be 'The pony's saddle' to demonstrate possession by one pony.