Identify the part of speech of the underlined word: The boys ran round the tree.

Answer: Preposition.

A preposition is a word that shows a relationship (such as direction, location, or time) between a noun or pronoun and other words in a sentence.

In the sentence "The boys ran round the tree," the word 'round' shows the relationship between the action 'ran' and the noun phrase 'the tree' — it indicates movement around the tree, so it functions as a preposition (equivalent to 'around').

Examples showing different uses of 'round':

• Adjective: The fruits are small and round. (here 'round' describes the noun 'fruits')

• Adverb: Everybody joins hands and dances round. (here 'round' modifies the verb 'dances' and does not take an object)

• Preposition: The earth moves round the sun. (here 'round' introduces the noun phrase 'the sun' and shows a relationship)

Therefore, in the given sentence, 'round' is used as a preposition.