(A) In given situation field at center is E towards A which due to charge qD
Now if charge B is removed then then field at center will be E towards A and E towards B
Now resulting field will be 2Ecosθ/2=2Ecos30=√3E
A->4
(B)If charge at C is removed then field at center will be E towards A and E towards C
resultant field 2Ecos2θ/2=E
B->2
(C)filed at center initially was due to charge at D now if it is removed then field will become zero.
C->3
(D)Since when charge at C is removed field at center is E towards B
and field at center due charge at B is balancing the field due to charge at E therefore, when charge at B is also removed then field due E will also have influence and which is towards B therefore, both the fields will add up and resultant field at center will be 2E
D->1