The most important factors for a credit score are on-time payments, credit history (i.e. length of time with accounts), and type of accounts. For the latter, this means credit cards, auto loans, home loans, student loans, etc. Lenders like to see diversity of credit types in your history. Credit cards are a very good place to start but by themselves probably won't get you into the maximum scores.

Less important factors, but still factors, include things like the number of accounts you have, the total credit limit on your accounts, and to an even smaller degree, the number of recent credit pulls.

Given this, he needs to start building a credit history. A credit card is a good way to do this and in the long run, I don't think it matters much if you do option a or b. I could be wrong, but even if you add him to your longest open credit card, he doesn't automatically inherit that credit history. His history starts at the point when he is added to the card. He would start with a larger credit limit which would benefit his score in the short run. But if they have a good income (or once they get one) and after a year or two of on-time payments, it's pretty easy to increase the credit limit on whatever credit card he goes with.

Also, it probably doesn't hurt to get a couple of different credit cards. I have several different kinds that have different rewards (some for travel, etc.). This increases the total number of accounts he has as well as the credit limit. He could also take out some other loans as others have suggested and then pay them off quickly.

All of this is assuming that he and your daughter are responsible and never really plan to carry a balance on their credit cards and just pay it off each month. If they would struggle with that, then that limits things.