Right answer is (a) pKa

The best I can explain: pKa is defined as the pH at which half of the groups are ionized. pKa is given by the equation, pKa = -log10[Ka], where Ka = Dissociation constant of an acid. Isoelectric pH (pI) is defined as the pH at which net charge on the molecule is zero. Ionic strength (I) is defined as half of the total sum of the concentration (ci) of every ionic species (i) in the solution times the square of its charge. Km is the substrate concentration at which half of the enzymes have bound substrate.