Which of the following sentences has CORRECT subject-verb agreement?

The sentence "Either kale or spinach is a healthy vegetable to eat with dinner" has correct subject-verb agreement.

The subject of the sentence is "Either kale or spinach". This is a compound subject, which means it is made up of two nouns. The nouns in a compound subject must agree with the verb in terms of number. In this case, both nouns are singular, so the verb must also be singular. The correct verb form is "is".
The other sentences have incorrect subject-verb agreement.
- The verb "kick" is plural, but the subject "One of the soccer players" is singular. The correct verb form is "kicks".
- The verb "was" is singular, but the subject "Several of the pieces of art" is plural. The correct verb form is "were".
- The verb "were" is plural, but the subject "The ambulance" is singular. The correct verb form is "was".