Final answer:
Threonine is less likely to form an alpha helix due to the absence of a hydrogen bond donor on its hydrophilic side chain, which is necessary for internal hydrogen bonding in the helix structure. Therefore, correct option is c.
Step-by-step explanation:
Threonine has a low propensity to form an alpha helix because it lacks a hydrogen bond donor on the side chain. Threonine is a polar amino acid with a hydrophilic side chain that includes a hydroxyl group (-OH).
This hydroxyl group makes it capable of forming hydrogen bonds, which are essential for stabilizing the alpha helix structure. However, the side chain itself does not have an additional hydrogen bond donor which would support the internal hydrogen bonding necessary for the alpha helix structure.
Other factors, such as steric hindrance or electrostatic interactions, can also influence an amino acid's propensity to be involved in an alpha helix, but in the case of threonine, it is specifically the absence of a hydrogen bond donor on the side chain.