Final answer:
The verb 'knows' is incorrect and should be changed to 'know' to match the plural subject 'few who visit the island'.
Step-by-step explanation:
The error with subject-verb agreement in the sentence is with the verb "knows". The correct verb should be "know", as the subject 'few who visit the island' is a plural noun phrase. Therefore, the plural form of the verb is required. Here is how the corrected sentence should read: 'Many tourists on the Hawaiian island of Oahu flock to the world-class beaches, but few who visit the island know about the Haiku Stairs, thousands of steel stairs that lead to one of the island's most impressive vistas.'
The incorrect verb in the sentence is 'knows'. The correct verb is 'know'. The subject-verb agreement error occurs when the verb 'knows' is used with the plural noun 'few'. Instead, the verb should be in the singular form 'know' to match the singular noun 'few'.'Many tourists on the Hawaiian island of Oahu flock to the world-class beaches, but few who visit the island know about the Haiku Stairs, thousands of steel stairs that lead to one of the island's most impressive vistas.'