Correct Answer - b
If `q` is charge on the earth and the moon, then
`1/(4piepsilon_(0))*(qxxq)/(r^(2))=G(M_(earth)xxM_(mo on))/(r^(2))`
or `q= sqrt(4piepsilon_(0)GM_(eart)M_(mo on))`
setting `G=6.6 xx 10^(-11)Nm^(2)kg^(-2), M_("earth")= 10^(25)kg`
and `M_("moon")= 10^(23) kg`, we get
`q= 8.57xx10^(13)C`