Answer
The order of stability of the carbocations is :
most stable $(d) \gt (c) \gt (b) \gt (a)$ least stable.
The carbocation $(b)$ is a secondary ($2^{\circ}$) carbocation.
Work Step by Step
The carbocation (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 carbocation.
$(a)$ is a primary carbocation, so it is the least stable.