Final answer:
The amino acid that is generally absent from an alpha-helix is proline (Option B).
Step-by-step explanation:
Proline is the amino acid that is generally absent from an alpha-helix due to its ring structure that disrupts the helix's regular geometry. The reason for this is that proline has an unusual ring structure that restricts its ability to fit into the regular geometry required for an alpha-helix formation.
In the context of an alpha-helix, amino acids like alanine are typically favored since they have smaller, less bulky side chains. Proline, on the other hand, introduces a kink in the helix, which disrupts the regular hydrogen bonding pattern necessary for the stability of the helical structure.
Thus, the correct option is B.