Alpha helix is a common secondary structure in proteins, characterized by a right-handed coil or spiral shape. This structure is stabilized by hydrogen bonds between the carbonyl oxygen atom of one amino acid residue and the amide hydrogen atom of another, which are located four residues ahead in the amino acid sequence.

The primary structure of a protein, which refers to the linear sequence of amino acids, determines the secondary structure. While all proteins have the potential to form alpha helices, some proteins are more likely to adopt this structure due to specific amino acid sequences or environmental factors.

In the given options, the protein that primarily has an alpha helix in its structure is Myoglobin (option A). Myoglobin is a globular protein found in muscle tissues and is responsible for storing and transporting oxygen. It consists of a single polypeptide chain folded into a compact structure. The tertiary structure of myoglobin is primarily composed of alpha helices, which are arranged in a globular shape to create a hydrophobic pocket for binding and storing oxygen molecules. The helical structure allows myoglobin to have a high degree of stability and compactness.

Silk fibroin (option B) is a fibrous protein primarily found in silk fibers produced by spiders and silkworms. It is composed of repeating amino acid sequences, with a high content of glycine and alanine residues. Silk fibroin is known for its beta-sheet secondary structure, which forms a highly organized and stable structure in silk fibers.

Ribonuclease (option C) is an enzyme that catalyzes the hydrolysis of RNA molecules. It is a globular protein with a complex tertiary structure. While it may contain some alpha helices, its structure is primarily composed of beta-sheets and loops.

Porin (option D) is a type of transmembrane protein found in the outer membranes of bacteria and mitochondria. It forms channels or pores that allow the passage of ions and small molecules across the membrane. Porins typically have a beta-barrel structure, where beta-strands form a cylindrical shape.

In summary, while all proteins have the potential to form alpha helices, myoglobin is primarily composed of alpha helices in its tertiary structure. Silk fibroin, ribonuclease, and porin have different structural motifs, such as beta-sheets or beta-barrels, which contribute to their unique functions.