Which is the correct order of stability for the given species?

Most stable free radical is 

${\mathrm{CH}}_3-\mathrm{C}\equiv \mathrm{C}-\mathrm{HC}=\mathrm{CH}-{\mathrm{CH}}_2-{\mathrm{CH}}_3$ 

       (a)             (b)                  (c)         (d)

Homolytic cleavage of which $\mathrm{C}-\mathrm{C}$ single bond requires the maximum energy?

The correct stability order for the following carbocations will be:

(I)     

(II)  

(III)  

(IV)