Which type of bonds is responsible for the strength and elasticity of hair?

The strength and elasticity of hair are primarily attributed to chemical side bonds. These bonds include disulfide bonds, which are a type of covalent bond that forms between the sulfur atoms in the amino acids cysteine, helping to create the structure of hair. This network of disulfide bonds contributes significantly to hair's overall strength and resilience.

Chemical side bonds play a crucial role because they are stronger and more durable than other types of bonds present in hair, such as hydrogen bonds and salt bonds. While hydrogen bonds are important for temporary styling and can affect the shape of hair under wet conditions, they are much weaker compared to chemical side bonds. Likewise, salt bonds, which are based on the attraction between charged groups in the hair's protein structure, can also influence hair's shape but are not as pivotal in overall strength and elasticity as disulfide bonds.

In essence, for hair to maintain its desired shape and resilience over time, the chemical side bonds that create a robust framework are indispensable. Thus, the correct answer highlights the key role of these strong chemical interactions in sustaining hair's structural properties.