The correct punctuation and subject-verb agreement for the sentence would be "A newly uncovered Viking fortress on Zealand, Denmark's largest island, is shaped like a ring and may have served as a military training ground for Viking attacks on England."
<h3>The correct punctuation for the sentence</h3>
The phrase "Denmark's largest island" functions as an appositive in the sentence, which means it renames the noun "Zealand," providing more information about it. Appositives such as this one, explaining a term in the sentence, should be set off by commas.
<h3>Subject-verb agreement</h3>
A verb should agree with the subject when it comes to being singular or plural. In the sentence, the subject "fortress" is singular, so there is no need to use the plural verb "are". The correct verb would be "is".
Learn more about subject-verb agreement here:
brainly.com/question/1835508
