Answer
$\tau = (-\frac{2.0}{\sqrt{t}}~N \cdot m)~\hat{k}$
Work Step by Step
Since the particle moves clockwise around the origin, by the right hand rule, the direction of the angular momentum is in the -z direction.
We can express the angular momentum in unit-vector notation:
$l = (-4.0~\sqrt{t}~~~kg~m^2/s)~\hat{k}$
We can find the torque that acts on the particle:
$l = (-4.0~\sqrt{t}~~~kg~m^2/s)~\hat{k}$
$\tau = \frac{dl}{dt} = (-\frac{2.0}{\sqrt{t}}~N \cdot m)~\hat{k}$
