A deer ___ in the forest.

The correct answer is 'lives'. Here's why: * The subject 'a deer' is singular (one deer). Therefore, the verb needs to be in the singular form as well. * 'Lives' is the third-person singular form of the verb 'to live'. * 'Live' is the plural form and would be used with a plural subject (e.g., 'Deer live in the forest'). * 'Don't live' implies a negative statement, meaning the deer does not live in the forest, which doesn't fit the context. So, the correct sentence is: A deer lives in the forest.