An alternate braking system is the use of a different hydraulic source for brakes than the normal braking system. Whereas brakes are hydraulically powered, and there are no alternate brake assemblies, it follows that if the normal braking system is lost, then another hydraulic source will be required to pressurize the brakes. It's the same brakes being used, just a different hydraulic source.
With this in mind, if one system fails, having another automatically take it's place may be unwise; this may cause the loss of the reserve or alternate braking system automatically too. Therefore, it should be pilot-selectable, and generally is.
Usually the alternate brakes and main brake hydraulic sources arrive at a common point before the brakes, which is a shuttle valve. If reserve braking and normal braking pressure is applied to the valve, the source with the highest metered pressure will open the shuttle valve, block the other system, and pressurize or power the brakes.
Anti-skid is just the opposite of braking; anti-skid is the dumping of brake pressure. When an anti-skid system determines that a locked wheel condition might occur, based on the rate of an individual or pair of wheels slowing down, it releases pressure to the offending wheel(s), dumping brake pressure, and preventing the wheel(s) from skidding. It follows, then, that anti-skid has nothing to do with activing alternate, or reserve brakes. In fact, often in alternate brakes, the braking capability, and anti-skid capability, is reduced somewhat.
During a normal landing, ground spoilers are deployed, causing loss of lift and putting more weight on the wheels, enabling more effective braking. During a normal landing, the weight on the nosewheel may control certain functions (such as pressurization, or other systems)...but not the reserve brakes. Whereas on every landing the spoilers will be deployed, and on every landing the nosewheel touches down, it follows that the reserve brakes aren't going to automatically be activated on every landing. Remember, reserve brakes are only needed when the normal braking is not available. Thing brings us back full circle to the concept that the reserve brakes should only be put in play when the pilot decides he or she need them, which makes it a pilot-selectable item.
To return to your question, then, the only answer that applies is A.