Find the error with subject-verb agreement. Select the incorrect verb and type it correctly. At the Naval Surface Warfare Center outside of Washington D. C a sophisticated indoor ocean that can recreate eight different open-water conditions are used to test models of
ships.

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.

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.

quite an interesting way to teach topic sentences, because the answer to this question is the best definition of a topic sentence that one can present, it is as if answering the questions taught you how to do it, this is some real teaching, it is terribly well-done.

the topic sentence in this paragraph is:

topic sentences writers stay on track and keep a paragraph from becoming an aimless collection of sentences about an idea.

which incidentally tells you what a topic sentence should be.

may i see the passage?