Some scientists have collected and stored seeds for many types of food-producing plants. the purpose of
this is to
(1) increase the destruction of environments (3) decrease the dependence on plants for food
(2) continue the deforestation of world ecosystems (4) preserve the diversity of plant species

If scientists collect and store seeds samples this means that they want to have a collection of the species with the objective of being sure that the species won't disappear- with the objective of preserving the species.

If they collect seeds of many species, this means that they're interested in the diversity of species: the correct answer is (4) preserve the diversity of plant species