During the past three years of its present administration, Country M's

Hi Kiara229,
You make a valid point. But, the thing that makes (D) inferior to (E) is the term, "value per item" in (D). Value per item does not tell you much about the increase or decrease in the value of total imports, something that (E) does clearly.
To expound further:
Say the import three years before was valued at $100, and say, there were 100 items imported. So, the value per item would be $1 per item. After three years, suppose the number of items decreased to a mere 10 items, but their price has increased from $1 per item to $10 per item. This would again make the total import value $100 dollars only. So, you see, there's no net change in the import value although the price per item has increased by 10 times. So, just by saying that the price per item has increased gradually, you can not really weaken the conclusion as much as tripled imports in (E) does. (E) doesn't leave that room of speculation whether the total imports value increased with respect to the increase in price per item, thus, it is a superior choice than (D) is.

In CR, you would often get down to two choices that might both make some sense to the question asked, but, you have to choose the one that makes the most sense. Or with respect to "weaken" type questions- Choose the option that hits the conclusion most negatively.

Hope it helps.