The correct answer is: A. The milk turned sour after sitting out on the counter all night.
There are verbs in the English language followed either by an adjective or a noun, and we call them linking verbs. Some of these verbs are: be, appear, look, remain, seem, and turn, like in the example above.
Linking verbs are verbs which link the subject with more information about it, or connect the subject with the sentence cemplement.