Hi,
what you are going to undermine here is the link between the food supply and the increase in human population. Indeed, you have to undermine the pace between these two factors.
Quote:
The statement that -The rate of population increase has begun to decline in Northern Europe, but the food supply has not diminished- weak-ens Malthus's postulate that food supply cannot keep pace with the rate of increase of human population.Likewise, if the human population growth may be limited by the use of contraception,- then it is possible that increasing the supply of food might not necessarily be followed by an increase in human population.
Notice how this question is incomplete, lacking one answer choice at least and
increase of human is wrong: you must use the preposition
in not of.
Hope this helps.
Note: from this, you can infer how is really difficult to find GRE worth material, even from renewable books such as Barron.