……. is an example of a heterogeneous mixture.
Sand in water
The explanation for the correct option
Option D, Sand in water
Heterogeneous mixtures have visible boundaries of separation between the various constituents. For example sand and water, sugar, and sand.
The explanation for incorrect options
A homogenous mixture does not have visible boundaries of separation between the various constituents. Examples are the salt solution, sugar solution, and soap solution.
Hence Option D, Sand in water is the correct option.