Which sentence uses a correct possessive noun? The tree' trunks are skinny. The trees's trunks are skinny. The trees trunks are skinny. The trees' trunks are skinny. The tree's trunks are skinny.

The sentence that uses a correct possessive noun is ‘The trees' trunks are skinny.’ The ‘trees’ represent authority or is in possession of its ‘trunk’. Also, the apostrophe is added after ‘s’ because the ‘trees’ already has an ‘s’ in it.

The correct sentence is "The trees' trunks are skinny"

First two are grammatically incorrect while the fourth would have you believe that one tree has multiple trunks.