Final answer:
The proper noun in the sentence is 'Plymouth'.
Step-by-step explanation:
In the sentence 'Plymouth rock is on the shore at the edge of Cape Cod Bay,' the proper noun is Plymouth. Proper nouns are the specific names of people, places, or things. They are capitalized to distinguish them from common nouns. In this case, 'Plymouth' refers to a specific location and is therefore the proper noun in the sentence.
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