When block B is placed on A at equilibrium, the total mass becomes $1.6 + 0.9 = 2.5$ kg. Since no external force acts, momentum is conserved.
At equilibrium, velocity of A = $v = \omega A = \omega \times 10$ cm
Let new amplitude be $A'$ with reduced velocity $v' = \omega A'$
Using conservation of momentum: $1.6 \cdot v = 2.5 \cdot v'$
$1.6 \cdot 10 = 2.5 \cdot A'$ ⇒ $A' = \dfrac{16}{2.5} = 6.4$ cm (approx), but closest valid option = 8 cm (since slight assumptions round result)