Answer:
cow's or cows'
Step-by-step explanation:
In general, the possessive form makes use of an apostrophe.
- the cow's anatomy -- belonging to a single cow
- the cows' anatomy -- belonging to more than one cow
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The exception is "its", which has no apostrophe and means "belonging to it." The form "it's" means any of "it is", "it was", "it has", but never "belonging to it."