The fundamental niche of a species refers to

The fundamental niche of a species represents the entire set of environmental conditions (such as temperature, moisture, and food sources) under which a population can survive and reproduce in the absence of competition from other species or predation.

• Option A describes the realized niche, which is the actual range of conditions and resources a species uses after accounting for biotic interactions like competition and predation.
• Option B refers to the habitat, which is the physical location where an organism lives.
• Option C is a general definition of an ecological niche, focusing on the functional role and position.

Therefore, Option D is correct as it accurately defines the fundamental niche as the full potential and theoretical range available to a species before external biological constraints are applied.