Verified Answer :

The question is asking us "Which of the following singular nouns does not form an irregular plural?". So let's see the plurals of the words:

AOx Plural : oxen
BTooth Plural: teeth
CMouse Plural: mice
DTree plural : trees. Trees is formed in a regular way, adding an "s" at the end - so this is the correct answer!

The answer is d. Hope it helps