The singular noun that does not form an irregular plural is option A: tree
In order to turn tree into the plural, you only have to add an -S.
All the remaining nouns are irregular
ox-oxen
tooth-teeth
mouse-mice
Irregular plural nouns are nouns that do not become plural by adding -s or -es, as most nouns in the English language do.
Many English words become plural by changing their vowels, such as oo to ee or an to en.