The sentence that is punctuated correctly is C, "The waiters, dressed in their white jackets, are already arranging the chairs on the sidewalk."
This sentence clearly separates unnecessary information from the sentence by using commas. The same rule should be applied to the other sentences. They should read as follows:
A. My mother's shoes, which cost her less because of a sale, looked superb with her evening gown.
B. The horses, spent from an entire day running on the track, slumped to their stables.
D. My friends, who often have shared interests with me, still keep in touch with me via social media.