Final answer:
The underlined word 'largest' in the given sentence functions as an adjective, providing more information about the noun 'selection'.
Step-by-step explanation:
The part of speech of the underlined word in the sentence 'We offer the largest selection of cool beverages, tasty meals, and luscious desserts in town.' is an adjective. In this sentence, 'largest' is the superlative form of the adjective 'large', which modifies the noun 'selection' to express the extent of variety the establishment offers. Remember, adjectives modify nouns by providing more information about them, and they can come in the positive, comparative, and superlative forms.