Is One Of These An Appropriate Substitute For A Stewing Hen For Making Chicken Stock? A.) Cornish Hen

B.) Baking Hen

C.) Capon 

Both Michel Roux and James Peterson advise using a stewing hen for making a chicken stock, but unfortunately no grocery stores in my area carry any. They do carry the three listed above tho.
From Poultry Meat Processing by Casey M. Owens and Christine Alvarado:


A stewing hen is the 52 week old breeder bird.

I can only guess to Roux and Peterson's suggestion, but it is probably based on size, and/or maybe that the maturity brings flavors to the stock that a young hen does not.

I would avoid the Cornish hen. They are very young and small.

Out of the options you list, you'll be OK with a large Baking hen.

Suggestion, if you have any Asian supermarkets nearby: 


check out their poultry, sometimes they have better than the factory fed chain stores
pick up some chicken feet and add them to your stock, they add a great color and flavor