Explanation
the force between 2 charges is given by the formula:
[tex]\begin{gathered} F=\frac{Kq_1q_2}{r^2} \\ where \\ q_i\text{ is the charge on object i} \\ r\text{ is the distance between the charges} \\ K=Coulomb\text{ constant=8.9875*10}^9N*\frac{m^2}{c^2} \end{gathered}[/tex]
then,we can check every option and compare the result to the original, hence
Step 1
a)option A
reduce one particle's charge by a factor of 16
so
[tex]q^{\prime}_1=\frac{q_1}{16}[/tex]
now, replace
[tex]\begin{gathered} F=\frac{Kq_1q_2}{r^2} \\ F_A=\frac{K\frac{q_1}{16}q_2}{r^2}=\frac{1}{16}\frac{Kq_1q_2}{r^2} \\ F_A=\frac{1}{16}(original) \\ F_A=\frac{1}{16}\frac{Kq_1q_2}{r^2} \end{gathered}[/tex]
therefore, option A works
Step 2