Answer: Option D
Step-by-step explanation:
When the pH of the soil changed from the optimum level to the stress level then only the stress tolerant earthworm would be left in the soil.
The earthworm that can tolerate the low pH will be common in the coming generation of the earthworm because only fittest will survive and reproduce.
Due to this reason the low pH tolerant population of the earthworm will be common.
So, the correct answer is option D