**Core Concept**
The definitive host of a parasite is the organism in which the parasite reaches its reproductive maturity. In the context of ascariasis, caused by the roundworm *Ascaris lumbricoides*, the definitive host is crucial for the parasite's life cycle.

**Why the Correct Answer is Right**
*Ascaris lumbricoides* requires a human host to reach maturity and reproduce. Humans are the definitive host because the parasite completes its life cycle within the human intestine, where it grows, matures, and lays eggs that are then shed in the feces. This process is facilitated by the human host's specific environment, including the gut pH and nutrient availability, which support the parasite's growth and reproduction.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the definitive host of *Ascaris lumbricoides* is not a non-human mammal. While other animals can be infected with *Ascaris* species, humans are the primary definitive host for *A. lumbricoides*.
**Option B:** This option is incorrect because the intermediate host of *Ascaris lumbricoides* is not the definitive host. The intermediate host, typically a beetle, is where the parasite's eggs hatch and the larvae develop before being ingested by a human host.
**Option C:** This option is incorrect because the definitive host of *Ascaris lumbricoides* is not a bird. While birds can be infected with *Ascaris* species, they are not the primary definitive host for *A. lumbricoides*.

**Clinical Pearl / High-Yield Fact**
The definitive host of *Ascaris lumbricoides* is humans, and the parasite reaches maturity and reproduces within the human intestine. This is an important distinction to make when considering the life cycle and transmission of the parasite.

**Correct Answer:** D. Humans are the definitive host of *Ascaris lumbricoides*.