Final answer:
Henry promises to wait for Keiko until the war is over.
Step-by-step explanation:
The promise that Henry makes to Keiko is option a. That he will wait for her until the war was over. In the given text, Henry expresses his love for Keiko and his intent to do the best he can for her while they are separated due to the war. He mentions that marriage seems impossible in the current circumstances, indicating that he will wait for her until the war is over