The correct answer is option 'A': R-groups.

Amino acids are the building blocks of proteins. There are 20 different amino acids that can combine in various ways to form different proteins. Each amino acid consists of a central carbon atom (called the alpha carbon) bonded to four different groups: an amino group, a carboxylic group, a hydrogen atom, and a unique side chain, also known as the R-group.

The R-group, or side chain, is what differentiates one amino acid from another. It can vary in size, shape, charge, polarity, and chemical properties. The R-group determines the unique characteristics and functions of each amino acid.

The side chain can be categorized into different groups based on its chemical properties. Some common categories of R-groups include:

1. Nonpolar side chains: These R-groups are hydrophobic, meaning they do not interact with water. Examples include glycine, alanine, valine, leucine, isoleucine, proline, methionine, phenylalanine, and tryptophan.

2. Polar side chains: These R-groups are hydrophilic, meaning they interact with water. Examples include serine, threonine, cysteine, tyrosine, asparagine, and glutamine.

3. Charged side chains: These R-groups are either positively charged (basic) or negatively charged (acidic). Examples of basic side chains include lysine, arginine, and histidine. Examples of acidic side chains include aspartic acid and glutamic acid.

4. Specialized side chains: These R-groups have unique properties that do not fit into the above categories. For example, cysteine can form disulfide bonds, and glycine is the smallest amino acid.

The R-group determines how an amino acid interacts with its environment and other amino acids in a protein. It affects the protein's structure, stability, and function. The specific arrangement and composition of amino acids in a protein determine its three-dimensional structure and ultimately its biological activity.

In summary, the 20 different amino acids have different R-groups, which are responsible for their unique properties and functions. The R-group determines how amino acids interact with their environment and other amino acids in a protein, ultimately influencing the structure and function of the protein.