Final answer:
The form of memory loss described is anterograde amnesia, where Xander is unable to recall any memories before his third birthday party (Option 3).
Step-by-step explanation:
The form of memory loss described in the question is anterograde amnesia. Anterograde amnesia is the inability to remember new information or events after a traumatic event or injury. It affects the formation of new episodic or semantic memories, while procedural memories are still able to be formed.
In Xander's case, his inability to recall any memories before his third birthday party indicates that the amnesia covers his whole life, from birth to the present. Hence, 3 is the correct option.