In a quaternary protein structure, what type of interaction would you most likely expect between the R groups of the following pair of amino acids: lysine and aspartic acid?

Hydrogen bonding due to their ability to form polar interactions at physiological pH

Covalent bonding via a disulfide bridge between the side chains

Ionic bonding due to electrostatic attraction between their charged R groups

Hydrophobic interactions because of their nonpolar R groups