Which of the following singular nouns changes vowels when made plural? A. Boy B. Sheep C. Bison D. Mouse

The question is asking us which of the following singular nouns changes vowels when made plural. Let's list the plural forms of the words: A. Boys B. Sheep C. Bisons D. Mice. We see that the plural of "boy" and "bison" is regular and that "sheep" and "mouse" have irregular plurals, but only D. Mouse forms its plural by a vowel change - answer is D.