Read the given sentences and choose the verbs that agree with the nouns.
­­­­Sam and his colleagues _____ organised a farewell party for their boss.

The correct answer is: Have

The correct answer is "Have".
Here, the sense of completing a work very recently is there as Sam and his colleagues just completed the process of arranging a farewell party for the boss. We know that present perfect tense is used in such case and its structure is "Has/Have + v3". Since the noun "Sam and his colleagues" is third person plural number, the correct answer is "Have" ("Has" is used for third person singular and for the rest, "have" is used).