Answer
$a_g = 9.84~m/s^2$
Work Step by Step
Note that at the crust-mantle interface, the mass of the crust does not affect the gravitational acceleration.
To find the gravitational acceleration at the crust-mantle interface, we can assume that all of the mass of the mantle and core is concentrated at the center of the Earth:
$a_g = \frac{GM}{R^2}$
$a_g = \frac{(6.67\times 10^{-11}~N~m^2/kg^2)(4.01\times 10^{24}~kg+1.93\times 10^{24}~kg)}{(6.345\times 10^6~m)^2}$
$a_g = 9.84~m/s^2$
