Hmm. What was the answer for this?
There's 4 different Hydrogens a radical can react with:
- Secondary (vinylic) - 1 Hydrogen
- Primary (vinylic) - 1 Hydrogens (Labeled I)
- Secondary (allylic) - 2 Hydrogens (Labeled II)
- Primary - 3 Hydrogens (Labeled III)
I'm not really sure how vinylic radicals rank in stability (maybe someone can clarify), but I do know a secondary allylic radical is more stable than a primary radical (same trend follows for carbocations).
In terms of reactivity vs. selectivity of halides:
Fluorine is explosively reactive
Chlorine which is moderately reactive
Bromine is more selective than reactive (generally prefers most stable product)
Iodine is very very selective and generally doesn't react.
If I had to answer this question, I would focus on choosing the radical environment with the most amount of Hydrogens available (most available to react with) rather than focusing on the stability. I think if it was Radical Bromination, then the more stable product would predominate -- but I don't think that's the case here.
Based on that logic, I'd rank them as follows:
Choice A: I (with 1 hydrogen) < II (with 2 Hydrogens) < III (with 3 Hydrogens)