The correct option is B 3.4mg
m1m2=E1E2 (By faraday law for same current and time)
Where E1 and E2 are the chemical equivalents and m1 and m2 are the masses of copper and silver respectively.
E=AtomicweightValencyE1=63.572=31.79 and
E2=107.881=107.88
∴1mgm2=31.79107.88⇒m2=107.8831.79mg=3.4mg