Potential at $10 \mathrm{cm}$ from surface
$=\frac{K Q}{R+10}=75 \Rightarrow R=10 \mathrm{cm}$
$\therefore$ Potential at surface $=\frac{K Q}{R}=\frac{100 \times 15}{10}=150 \mathrm{V}$
Electric field on surface $=\frac{K Q}{R^{2}}=\frac{100 \times 15 V \times c m}{100 c m^{2}}$
$=1500 \mathrm{V} / \mathrm{m}$

$I.$ Energy stored in ${C_1}$ $=$ Energy stored in ${C_2}$ $+$ Energy stored in ${C_3}$
$II.$ Charge on ${C_1}$ $=$ Charge on ${C_2}$ $+$ Charge on ${C_3}$
$III.$ Potential drop across ${C_1}$ $=$ Potential drop across ${C_2}$ $=$ Potential drop across ${C_3}$
Which of these is/are correct