Which mixture can be classified as a heterogeneous mixture ?
a. steel
b. iron
c. 14 karat gold
d. salad dressing

Salad dressing is considered a heterogenous mixture because the components of the mixture are not evenly distributed throughout.

Not Salad dressing? Is that what you think? HaHa 
A heterogeneous mixture is made up of two or more substances which cannot mix completely together, so the composition is variable and you are readily able to identify both type of substances with observation.
Iron is only one substance. Its D. Salad dressing because you are able to see other colors in salad dressing. Like many different herbs, flavors and so on. A heterogeneous mixture is made of different substances that remain physically separate, so I would say salad dressing. D is a Heterogeneous Emulsion or Suspension.