You answer a) is the correct one.
The builder gives you the opportunity to take the build action on another players turn by paying 2 coins, therefore allowing you a different option when your turn comes back round again.
You still must have (and pay) the required resources.
The builder gives you the opportunity to take the build action on another players turn by paying 2 coins, therefore allowing you a different option when your turn comes back round again.
You still must have (and pay) the required resources.