**Core Concept**
The underlying principle being tested is the concept of **eugenics**, which involves the practice of improving the human species by selectively mating people with specific desirable traits. This concept is rooted in **genetics** and **social Darwinism**.

**Why the Correct Answer is Right**
The statement describes the ideology of **Nazi eugenics**, which aimed to "purify" the German race by eliminating individuals deemed weak or defective. This was achieved through **forced sterilization** and **euthanasia** programs. The goal was to promote the survival and reproduction of those with desirable traits.

**Why Each Wrong Option is Incorrect**
**Option A:** This choice is incorrect because it does not relate to the context of the statement.
**Option B:** This choice is also incorrect as it does not pertain to the concept of eugenics.
**Option C:** This option is incorrect because, although it may relate to genetics, it does not specifically describe the intention of killing the weak and defective.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that **eugenics** has been widely discredited due to its association with **human rights abuses** and **genocide**.

**Correct Answer:** D. Eugenics.