Final answer:
A brass key represents a heterogeneous mixture as it is an alloy made of copper and zinc, which are two distinct metals.
Step-by-step explanation:
A heterogeneous mixture is a combination of different substances that are not evenly distributed throughout, meaning you can visibly see the different components. Out of the options provided, the object that represents a heterogeneous mixture is the brass key. Brass is an alloy made by combining copper and zinc and it appears as a mixture of two distinct metals.