You want to decorate the bottom of an aquarium with brightly colored rocks of different shapes and sizes. this describes a

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Below are the choices:

homogenous mixture 
heterogeneous mixture 
semi mixture

You want to decorate the bottom of an aquarium with brightly colored rocks of different shapes and sizes. this describes a homogenous mixture 

The word hetero means different. Since the rocks are varied i assume it would be a heterogeneous mixture.

The answer is option a