Which amino acid appears in the mutated protein sequence replacing proline?

The correct choice indicates that threonine replaces proline in the mutated protein sequence. Amino acids can undergo mutations, which can result in the replacement of one amino acid with another during protein synthesis. Proline is characterized by its unique cyclic structure that contributes to the folding and stability of proteins. When threonine is introduced as a replacement, it radically alters the properties of the protein, since threonine is hydrophilic and has a different side chain structure. This change can affect the protein's overall structure, function, and interactions within biological systems.

Understanding the nature of amino acid substitutions is crucial, as it highlights how minute changes in genetic coding can lead to significant ramifications in protein functionality, which is central to the study of biology and genetics.