In a balanced Wheatstone bridge with resistors $R_1, R_2, R_3, R_4$ (galvanometer across the B-D diagonal), the balance condition for zero galvanometer current is

1. At balance, no current flows through the galvanometer ($I_g = 0$). 2. Applying the loop rule to the two loops gives $I_1 R_1 = I_2 R_2$ and $I_1 R_3 = I_2 R_4$. 3. Dividing the two relations yields $R_1/R_2 = R_3/R_4$. 4. So A; the product and sum forms (B, C) are not the balance condition. _Source: NCERT Class 12 Physics Ch 3 "Current Electricity", p.20_