The correct answer is option D, which means that all three factors - habitat loss, frequent poaching, and the construction of windmills - contribute to the declining population of the Great Indian Bustard.

1. Habitat Loss:
- The Great Indian Bustard is a grassland bird that requires vast open grassland habitats for breeding, feeding, and nesting.
- Over the years, due to urbanization, agricultural expansion, and industrialization, these grasslands have been converted into human settlements, agricultural fields, and infrastructure development.
- This loss of habitat has resulted in the fragmentation and degradation of the remaining grasslands, making it difficult for the Great Indian Bustard to find suitable areas for survival.

2. Frequent Poaching:
- The Great Indian Bustard is a highly valued bird in the illegal wildlife trade due to its large size, distinctive appearance, and cultural significance.
- It is hunted for its meat, feathers, and other body parts, which are used in traditional medicine and for ornamental purposes.
- The illegal poaching of Great Indian Bustards has significantly contributed to their declining population, as it directly removes individuals from the wild and disrupts their reproductive success.

3. Construction of Windmills:
- Wind energy is being promoted as a clean and renewable source of energy, leading to an increase in the construction of wind farms across the country.
- Unfortunately, many of these wind farms are located in the same grassland habitats where the Great Indian Bustard resides.
- The construction and operation of windmills pose a significant threat to the bird, as they can collide with the moving blades of the wind turbines, resulting in fatal injuries.
- The presence of wind farms also disrupts the bird's natural behavior and habitat use, further impacting their population.

In summary, the critically endangered status of the Great Indian Bustard is primarily due to habitat loss, frequent poaching, and the construction of windmills. To conserve this species, it is crucial to protect and restore its grassland habitats, enforce strict laws against poaching, and carefully plan the location and design of wind farms to minimize their impact on bird populations.