Final answer:
The underlined word 'Supreme Court' is a proper noun acting as an adjective in the sentence.
Step-by-step explanation:
In the sentence 'Sandra Day O’Connor is a retired Supreme Court judge,' the underlined word 'Supreme Court' is a proper noun acting as an adjective. Proper nouns are specific names of people, places, or things, and they can act as adjectives to describe other nouns. In this case, 'Supreme Court' is describing the noun 'judge' to specify the type of judge referred to.