Answer:
The sentence that has an error in subject-verb agreement is:
His discoveries was a combination of luck and hard work.
Explanation:
When we say a subject and a verb have to agree, we mean the verb must be inflected in accordance to the subject. If the subject is singular, the verb must also be singular. If the subject is plural, the verb is also plural. The verb "to be", especially, is quite an irregular verb, and requires due attention.
Having that in mind, we can say the sentence that contains an error is "His discoveries was a combination of luck and hard work." The noun "discoveries" is the main word of the subject, and it is plural. However, the verb "was" (past form of the verb "to be") can only be used for singular subjects. For the sentence to be correct, it should be:
His discoveries were a combination of luck and hard work.
