Answer:
A. Deer.
Explanation:
Though in normal grammar rules and norms to add 's' or 'es' in any plural forms of a noun, there are also exceptions. Though words such as 'bag, toy, apple, chair' etc have their plural forms as 'bags, toys, apples, chairs", there are also exceptional cases where this form don't apply. Words such as deer, information, news, sheep etc have their singular and plural forms remain the same.
The plural form of 'deer' is 'deer' but there is also another view of an 'accepted' plural form of deer which is 'deers'. But in general and in majority, the plural form of 'deer' is 'deer'.
Thus, the correct form of the word deer is deer.
