Alpha helix is a coiled structure stabilized by:
a. covalent interactions within a polypeptide strands
b. hydrogen bonding within polypeptide strands
c. gravitational attraction of bulky r groups
d. hydrogen bonding between polypeptide strands
e. hydrophobic interactions between polypeptide strands

B.hydrogen bonding within polypeptide strands

Explanation:

Alpha helix is one of the secondary structures formed by polypeptide chains. It is formed when the backbone of a polypeptide is wound around a longitudinal axis. The side chains of the amino acids are oriented outwards. Alpha helix structure is stabilized by the formation of a maximum number of hydrogen bonds between the amino acids of the chain. The hydrogen atom linked to the electronegative nitrogen atom of a peptide bond form hydrogen bond with the electronegative carbonyl oxygen atom of the fourth amino acid.

In this way, every peptide bond except the ones present close to each end of the helix take part in hydrogen bonding and stabilize the structure.