Which state of matter has a define shape and definite volume gas plasma solid liquid

A solid has a definite shape and volume

Off all the states of matter, only solids have a defined shape, due to stronger bonds between its molecules/atoms. Unlike solids, Liquids and gasses take the shape of whatever container they are placed into. This, again, is due to weaker linkages between the atoms/molecules of gasses and liquids.

d.7.5cm is your answer

electric field is a function of 1/r^2

yes, the force per unit charge for a point charge is inverse square law.