Which sample of matter represents a mixture?

A mixture is a new substance that you get when you mix two or more different substances together and blend them so you either create a homogeneous mixture, or a heterogeneous one. In you case (assuming that your choices are aqueous ammonia, liquid mercury, gaseous ethane, solid iodine), the correct answer is aqueous ammonia, given that it is a mixture of ammonia and water, whereas the other options are not mixtures because they only consist of one element.