Two point particles, with charges of $q_1$ and $q_2$, are placed a distance r apart. The electric field is zero at a point P between the particles on the line segment connecting them. We conclude that:
$q_1$ and $q_2$ must have opposite signs and may have different magnitudes
$q_1$ and $q_2$ must have the same magnitude and sign
$q_1$ and $q_2$ must have equal magnitudes and opposite signs
P must be midway between the particles
$q_1$ and $q_2$ must have the same sign but may have different magnitudes