Principle involved

Stability of carbon radicals formed by homolytic cleavage of a $\mathrm{C-H}$ bond depends mainly on:

• Hyperconjugation
• Resonance stabilization
• Degree of substitution

General order for simple alkyl radicals is: $3^\circ > 2^\circ > 1^\circ > CH_3$

Identify the nature of radicals at positions I, II, III, IV

From the given structure (X), the labeled hydrogens correspond to carbons which on homolytic cleavage give radicals of different substitution.

• Radical at II is the most substituted and hence gets maximum hyperconjugative stabilization.
• Radical at I is the least substituted / least stabilized among the four.

Compare all labeled positions

• Position II gives the most stable radical.
• Position I gives the least stable radical.

Match with options

The pair "most stable, least stable" is: $\boxed{\text{II, I}}$

This corresponds to Option D.

Verification with stored answer

Stored correct answer = D

Our derived answer = D

Hence, the answer agrees with the stored correct answer.