Explanation:

- Option A: "A drop in the mercury column" is correct. It refers to the decrease in the height of the mercury in a barometer, indicating pressure change.

- Option B: "of a barometer" is correct. The phrase specifies the instrument involved, which is appropriate in this context.

- Option C: "indicates the change in atmospheric pressure" is correct. It accurately describes the result of the drop in the mercury column.

- Option D: "No error" suggests there are no mistakes in the sentence.

Correct Answer: Option 4 - d