A. The deer eats the berry of the mistletoe.
B. Deer eat berries of mistletoes.
C. (The) deer eat(s) (the) berry(es) of (the) mistletoe(s).
B is the way I would usually write a sentence of the given meaning. The use of the, the definite article, to refer all the things which the noun concerns, is not familiar with me - the reason I am having trouble with A and C.
In C, how would you choose the articles? The most common way and, if any, the most formal way.
What if there is chance of misleading because there is already something to which the + singular may refer?