The correct answer is D) Grassland habitats.
Long-billed curlews are shorebirds that thrive in open grassland habitats where they can find food sources such as insects and other invertebrates. Increasing grassland habitats would provide better breeding and foraging opportunities for these birds, potentially leading to an increase in their population.
Pesticide use (A) typically has negative effects on bird populations due to loss of food sources and direct toxicity. Milkweed (B) is more beneficial for species like monarch butterflies rather than curlews. Forest habitats (C) are not suitable for curlews, as they prefer open areas.