Final answer:
The adverb in the sentence is 'deeply,' which describes the extent of Mr and Mrs Marshall's devotion to each other.
Step-by-step explanation:
The question asks to identify the adverb in the sentence: "Mr and Mrs Marshall, who have been married for fifty years, are deeply devoted to each other." The adverb in this sentence is deeply, which modifies the verb 'are devoted' by explaining to what extent they are devoted to each other.
The concept of devotion is further enriched by the context from various literature excerpts where characters express a profound sense of connection, like in the lines "Being married feels different" or "the love is deeper." These illustrate the depth of feeling that the word 'deeply' conveys in the original sentence.