The plural of deer is deer. This word is an irregular plural noun The noun deer is both the singular and the plural They saw one deer or They saw two million deer Deer is the preferred plural form of deer, although deers (rarely used) is also an accepted plural.

Hint: Deer is a four-legged animal. Some nouns do not change when pluralized.Complete answer: There are many words in the English language which are used unaltered to refer to singular or plural nouns. For e.g., Sheep, series, news, etc. These words refer to both singular forms of the noun as well as the plural form as needed. Deer is one such word too. It is used as per the requirement for both singular as well as plural. Thus, option d, Deer is the correct answer. Option b, dears is the plural of the word dear which is used as an affectionate or friendly form of address. For e.g. Dear mother or ‘My dears’ (used to refer to a group of people who are dear to someone). Hence, option b is incorrect. There is no such word as ‘deeres’ in the English language, therefore, option c too is incorrect.Note: To refer to more than one deer, we can say, two deer, ten deer, 100 deer, etc. we do not say two deers, or ten deers as that would be incorrect.

It's a matter of historical origin and subsequent development.

In the oldest recorded English deer belonged to the neuter declension, which did not have a distinct plural ending in the nominative and accusative cases. (It is believed that this declension did have plurals in Proto-Germanic, but they disappeared before English or any immediate ancestor was written down.) At that time there was no ambiguity, since the determiners accompanying these nouns did change in the plural.

Later, when the Old English endings were mostly lost, the majority of these neuter nouns acquired 'regular' plural endings in -n, eventually superseded by endings in -s: wīf, for instance, became wives in the plural. A few, however did not, and deer is one of these.

It is often remarked that all these nouns with invariant plurals denote animals, deer, sheep, fish, swine, which are either herded or hunted; and it has been suggested that both the 'mass noun' sense with herd animals and the custom of referring to all hunted animals in the singular (we hunt bear, lion, and elephant as well as deer) helped inhibit plural regularization.

ADDED: See the second edition (1954) of Jespersen, A Modern English Grammar on Historical Principles, Part II SYNTAX (First Volume), Ch.III The Unchanged Plural (pp. 49–69), especially 3.1–3.2 and 3.71.

Deer. however I've heard people say reindeers. I don't believe it's correct.