Among the options provided, the mixture that is considered heterogeneous is c) oil and water.
To explain further, a heterogeneous mixture is one in which the components can be distinguished and are not uniformly distributed. In the case of oil and water, when mixed, the oil forms distinct droplets or a separate layer because it is less dense than water and does not dissolve in it. This separation creates visible boundaries, making it easy to identify the different substances.
On the other hand, a) granite is a type of rock that contains different minerals, but it is still considered a rock rather than a mixture in this context. b) brass is an alloy composed of copper and zinc, blended to form a uniform material, thus it is homogeneous. Lastly, d) salt and water create a solution when salt dissolves in water, resulting in a homogeneous mixture where the salt cannot be visibly distinguished from the water.