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.
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Sign up to join this communityFrom 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:
Good information in Paulb's answer. However, you won't get the same flavor or depth of flavor from the baking hen as you will from a stewing hen. (There's a lot of difference between an 8-10 week old bird and a 52+ week old bird.)
In the US, stewing hens are usually sold frozen. I haven't seen a fresh one since I was a child. (Over 40 years.) So you may keep this in mind for future searches.
Re adding chicken feet, I have never tried it, but it may be worth trying.
What we do is use chicken gizzards, either boiled alone or added to a chicken stock pot. (We pre-cook gizzards for frying.) The stock from gizzards cooked alone is amazingly close to that from a stewing hen, with possibly even a little more depth of flavor.
The question is what do you want to acchieve.
If you want to maximize flavour, you need to realize that you will only be able to taste what went in and an older bird, possibly even one that lived free-range and had a varied diet will have developed mature muscles and denser bones than what is - basically - a half-mature bird, raised in a short time on a less varied diet and in a sheltered environement. On a first glance, a stewing hen might look scrawny, the skin and meat will be darker and the fat more yellow and of a more intense flavour than what you are probably used to. Also note that for maximum flavour those chicken parts that are often discarded in Western cuisine (because they have little meat) are an excellent choice, from neck to wingtips and feet, gizzards and hearts. The meat, on the other hand, might be tougher than what you are used to (ymmv) but very flavourful.
But if you only have access to young birds and your goal is the stock, let me suggest a more frugal aproach: buy whole birds for your standard everyday cooking, chop them up yourself, use breasts and legs as usual and collect the carcasses and other trimmings for stock. A chicken breast from an 8-weeks old bird is delicious on its own, but pretty useless if you want to make stock. Again, adding extra necks, feet, gizzards if you need more chicken makes sense. If your standard store doesn't offer them, check out Asian stores, for example.