Identify what stabilizes an $\alpha$-helix

The $\alpha$-helix is a common secondary structure of proteins. Secondary structures in proteins are mainly stabilized by hydrogen bonds.

Nature of bonding in an $\alpha$-helix

In an $\alpha$-helix, the hydrogen bond forms between: $\text{C=O of one peptide bond and N-H of another peptide bond}$ specifically along the same polypeptide chain.

Check each option

• A: Covalent bonding
  Covalent bonds form the primary structure of proteins (linking amino acids), but they do not specifically stabilize the $\alpha$-helix.

• B: van der Waals forces
  These may contribute slightly to overall protein stability, but they are not the main interaction responsible for $\alpha$-helix stabilization.

• C: Ionic bonding
  Ionic interactions can stabilize tertiary structure in some proteins, but they are not the defining stabilizing force of the $\alpha$-helix.

• D: Hydrogen bonding
  This is the correct interaction responsible for stabilizing the $\alpha$-helix.

Conclusion

Therefore, the correct option is: $\boxed{\text{D: Hydrogen bonding}}$