**Core Concept**
The structure of alpha-helix is a fundamental aspect of protein secondary structure, characterized by a spiral conformation stabilized by hydrogen bonds between the carbonyl oxygen of one amino acid and the amide hydrogen of another, four residues away. This structure is sensitive to changes in amino acid composition, particularly the chirality of amino acids.

**Why the Correct Answer is Right**
When L-alanine is replaced by D-alanine alternatively, the resulting protein would have a mix of L- and D-amino acids. The alpha-helix structure relies on the regular arrangement of L-amino acids, which creates a left-handed spiral. However, the presence of D-amino acids disrupts this regularity, causing the alpha-helix to unwind and adopt a more random coil structure. This is because D-amino acids have a mirror-image configuration compared to L-amino acids, leading to steric clashes and altered hydrogen bonding patterns. As a result, the structure of alpha-helix is significantly altered, leading to a loss of its characteristic stability and function.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because replacing L-alanine with D-alanine would not simply cause a minor perturbation in the alpha-helix structure. The presence of D-amino acids would have a more significant impact on the overall structure.
**Option B:** This option is incorrect because the replacement of L-alanine with D-alanine would not lead to a stabilization of the alpha-helix structure. In fact, the opposite is true, as the presence of D-amino acids would disrupt the regularity of the alpha-helix.

**Clinical Pearl / High-Yield Fact**
The chirality of amino acids plays a crucial role in determining the structure and function of proteins. Understanding the impact of D-amino acids on protein structure is essential in fields such as protein engineering and drug design.

**Correct Answer: D. The structure of alpha-helix is significantly altered, leading to a loss of its characteristic stability and function.**