Answer:
At the Naval Surface Warfare Center outside of Washington D.C, a sophisticated indoor ocean that can recreate eight different open-water conditions is used to test models of ships.
Step-by-step explanation:
The subject-verb agreement, simply said, represents the grammar rule where subject and verb must agree in number, so if the subject is singular, the verb must also be in singular and vice versa - if the subject is plural, then the verb must be plural too.
In the given sentence we have the subject: a sophisticated indoor ocean - which is singular and we have the verb: are - which is plural, so they do not agree in number, so we have to change the number of the verb in order to have the subject-verb agreement completed.