Which amino acids contain positively charged/basic side chains?

The correct choice indicates the amino acids that possess positively charged or basic side chains. This characteristic is primarily due to the functional groups in the side chains which can accept protons, thereby carrying a positive charge at physiological pH.

Lysine contains a long aliphatic side chain with an amino group that becomes protonated, giving it a positive charge at physiological pH. Arginine has a side chain that includes a guanidino group, which readily accepts protons, making it positively charged as well. Histidine, while it is often considered a neutral amino acid at physiological pH, has a side chain that can either be positively charged or neutral depending on the pH, particularly in protein environments where it can play key roles in enzyme active sites.

In contrast, the other options consist of amino acids that do not have positively charged side chains. Tryptophan, tyrosine, and phenylalanine contain aromatic side chains that do not confer a positive charge. Cysteine, threonine, and serine, on the other hand, have polar side chains with sulfhydryl and hydroxyl groups, respectively, but also do not carry a positive charge. Lastly, alanine, valine, and is