**Core Concept:** Atavism is a phenomenon where an organism exhibits traits characteristic of an earlier stage in its evolutionary lineage. These traits reappear due to genetic mutations or recombination, often in response to specific environmental pressures.

**Why the Correct Answer is Right:** Atavism refers to the reappearance of ancestral or primitive traits in modern organisms. In this case, the correct answer is "D." Atavism is the reappearance of ancestral traits in response to environmental pressures. This can lead to the expression of traits that were last seen in a more primitive organism or stage in the evolutionary lineage.

**Why Each Wrong Option is Incorrect:**

A. This option is incorrect because it refers to the concept of heterosis, also known as hybrid vigor, which describes the enhanced growth or vigor of offspring resulting from a cross between parents with diverse genetic backgrounds. Atavism involves the reappearance of ancestral traits, not increased vigor.

B. This option is incorrect because it is the term for the suppression of certain genes during embryonic development, causing malformations or developmental abnormalities. This is not related to the reappearance of ancestral traits.

C. This option is incorrect because it refers to the process of sexual reproduction, which involves the combination of genetic material from two parents to produce offspring. Atavism describes the reappearance of ancestral traits in the offspring, not the process of reproduction itself.

**Clinical Pearl:** Understanding atavism is crucial for understanding evolutionary pressures and how they can lead to the expression of ancestral traits in modern organisms. This concept is particularly relevant in medical contexts such as genetic disorders or medical conditions that may have an evolutionary basis.

**Correct Answer:** D. Atavism refers to the reappearance of ancestral traits in response to environmental pressures. This concept is important for understanding the relationship between evolution, genetics, and the expression of inherited traits in modern organisms.