Answer:
The correct answer is option c, that is, variation.
Step-by-step explanation:
In the given case, the gravel mix is to be selected on the basis of the size of pebbles, which is analogous to the organism selected in nature. In the given question, the sorting machine is analogous to the environment, that is, a selective force.
Like a specific environment permits the organisms exhibiting specific traits. For example, in a desert where the environment is hot and dry, only the species exhibiting the tendency to conserve water can thrive, similarly sorting machine will permit to distinguish gravel on the basis of the pebble size. Thus, different size of pebble resembles variation.