The option that best describes the word 'adorn' as it is used in the sentence is B) to decorate.
<u>'Adorn' means to add ornaments to a thing in order to enhance its appearance and make it more attractive</u>. In this case, the thing that is being decorated is the bridge. Moreover, in this sentence, <u>the sentinels are equated to ornaments because they are standing still near the stream as if they were statues</u> adorning the bridge.