Answer:
(B) 15 N.m
Explanation:
The deceleration of the wheel is first found by using the following formula:

where,
α = angular acceleration = ?
ωf = final angular speed = 0 rad/s
ωi = initial angular speed = 12 rad/s
t = time = 0.4 s
Therefore,

here, the negative sign shows deceleration.
Now, we find the torque responsible for this deceleration:

where,
τ = torque = ?
I = rotational inertia = 0.5 kg.m²
Therefore,

τ = 15 N.m
Therefore, the correct answer is:
<u>(B) 15 N.m</u>