33a - 2
they cancel each other.
b-3,4
Net charge in the ring is zero so potential zero at center.. (all charges are at same distance..)
I and IV quad has field in -i direction (away from them) and quad II and III has towards them -i (there is symmetry in charge direction so only -i component no j component..)
c-3,1
same reason as b
d-2
as it is proportional to sin2θ always positive so field is zero as in a (here also charge is symmetrically distributed..) But potential cannot be zero as only +ve charge..