From the words or groups of words lettered A to D, choose the word or group of words that best completes the sentence. One of his greatest achievements...th...

Answer Details

The option that best completes the sentence is "was". The subject of the sentence is "one of his greatest achievements", which is a singular noun phrase. Therefore, the verb that follows should also be singular. The past tense of the verb "to be" is "was", which agrees with the singular subject. The other options, such as "were", "have been", and "are", are all plural forms of the verb "to be" and do not agree with the singular subject.