Answer: The correct answer is option (c).
Explanation:
The thickness of the insulation needed for a wire with voltage E, th expression is given as:
[tex]0.01E+0.003E^2 mm[/tex]
The thickness of the insulation required for the 7 kilovolts : E=7 kilovolts
[tex]0.01(7)+0.003(7)^2=0.217 mm[/tex]
0.217 mm thickness of insulation material is needed for 7 kilovolt cable.
Hence, the correct option is (c).