Choose a. I did the math. Let the charge band x, then the force between a and o is f = kqx/L 2. Let the angle between AO and ao' (the projection point o' of this point on ABC is α. 3F * sinα= mg. Sinα can be calculated as: (the square root of 6) /3 by mathematical knowledge through a regular tetrahedron with a side length of L. From this, the answer of x=A can be calculated.