Answer
The order of stability of the radicals is :
most stable $(d) \gt (c) \gt (b) \gt (a)$ least stable.
The radical $(d)$ is a tertiary ($3^{\circ}$) radical.
Work Step by Step
The radical (d) is $3^{\circ}$ and resonance stabilized, so it is the most stable.
$(c)$ is also tertiary, so it is the next most stable.
$(b)$ is secondary, it is the next stable radical.
$(a)$ is a primary radical, so it is the least stable.