The substituents ${\mathrm{R}}_1$ and ${\mathrm{R}}_2$ for nine peptides are listed in the table given below. How many of these peptides are positively charged at $\mathrm{pH}= 7.0?$

At its isoelectric point, the amino acid remains entirely in Zwitter ionic state. Amino acids with an extra acidic group have an isoelectric point below 7.0 and become anionic when the pH rises over that threshold.

On the other hand, amino acids containing an extra basic group have an isoelectric pH greater than 7.0, and adding acid solution to lower the pH below the isoelectric point transforms them into cationic molecules. Since ${\mathrm{R}}_1$and ${\mathrm{R}}_2$are basic, the above peptide will stay protonated (cationic) when pH =7.0

4 is the right integer, therefore.