MM stated in the OP ‘Legions of Old’. So to look at the exam question I have decided to give some thoughts as to the original loyalist legions and how easy it would be for each to reconstitute. Whilst I will not say how hard it would be I will give a yes/possibly/no imho. In order of ease:
1. Ultramarines. Many successor chapters, respecting their originators, shared gene-seed, huge industrial political base in Ultramar. The easiest sell for me. I’ll give it a yes.
2. Space Wolves. Refusal to split down post heresy. 1 second founding doomed to failure. Now, I know we argue about numbers, but according to lexicanum, Thousands Sons numbered 10k during heresy. Conventional military wisdom says you attack in 3:1 odds. So lets say 30k SW attacked Prospero in order to win. lexicanum says Wolf Brothers got half the SW fleet, stands to reason half the men. Leaving 15k. That matches lexicanum comments that a Great Company is like a Chapter – so 12 great companies – 12-15k (Great Wolf company is bigger). So Space Wolves are a yes for me and arguably already at legion strength beyond the smallest crusade legions.
3. Imperial Fists. Loads of second founding and successor chapters. Dead loyal to the imperium, so they would obey an order to merge. Black Templars already at massive increased strength compared to normal chapters. I’d say yes, Even if you said 50% of the successor chapter were willing/able to join and even if you count BT as 1k in strength, they would have 12k marines – again a smaller legion but beyond that of the smallest crusade legion.
4. Dark Angels. Masses of successor chapters, all loyal to the primogenitors based on their shared secret. I reckon it as a strong possibility – they may feel that en masse their actions chasing the fallen would be subject to too much scrutiny.
5. Blood Angels. Undeniably loyal but also flawed. Again, would they want this many flawed marines with the curse of Sanguinius in one band? Having said that, it would give them a scale of economy previously missing in trying to solve the problem. They do have quite a few successor chapters and banding together thus curbing the excesses of some like the flesh-tearers, would maybe take some of the external heat away from them. I would rate them as possibly.
6. White Scars. Lots of successors, minor deviance to codex, I rate them a possibly based on the fact they are clearly Mongol/Genghis Khanesque and as such would probably dig a horde.
7. Iron Hands. Small number of successors, quite tetchy, already lost a 3rd of the chapter to form Sons of Medusa in a schism. I reckon they would never go along with it – No.
8. Raven Guard. Very small numbers post heresy, although quite a few successors. Slowly degenerating geneseed. I reckon a no.
9. Salamanders. Highly divergent geneseed, small numbers after heresy, no known successors. A 7 chapter formation based on noctunian culture, I can’t see it expanding so no.