This involves failing to take into account the whole (weight of a package) rather than the weight of a part. (A) would have been correct if it had said “weight of the packing material when determining the weight of the contents of a package based on the weight of the package as a whole.”

This doesn’t involve failing to take into account a part. The monetary value of the coins in a pile is similar to the whole — so (B) is more similar to failing to take into account the whole.

This doesn’t involve failing to take into account a part. The magnification of a lens isn’t something that can be subtracted from a whole in the way that the effect of El Nino can be subtracted from the change in global temperature.

(D) involves the potential use of false data as part of determining the average annual number of crimes committed. This isn’t analogous, because the temperature change attributable to El Nino isn’t something false or fake — it legitimately affects the temperature, but is something we want to exclude if we’re trying to determine the effect of a volcanic eruption. (D) would have been correct if scientists were trying to exclude false, fake, or unreliable data from our determination of the magnitude of a change in temperature.

To determine whether a change in average age is attributable to a change in number of births, we need to exclude the impact of another potential factor — the ages of new immigrants to the country, which would also affect average age. This is analogous to the need to exclude the impact of El Nino when determining how much of a temperature change is attributable to a volcanic eruption.