What is an example of a heterogeneous mixture ? a) gasoline b) diet soda c)grape juice d) flour and water

The answer would be D) flour and water 

To make it easy, you can generalize that a clear fluid is homogenous. Gasoline, juice and soda is a clear solution.
A heterogeneous mixture is not mixed completely. That means you can differentiate the solute with the solvent.
In this case, flour and water don't mix evenly because you can see the flour. If you keep the solution, some of the flour will be pulled by gravitation and create sediments. This kind of solution is called suspension.