Final answer:
The correct answer is three days.
Step-by-step explanation:
The question is asking how long it had been since Henry received a letter from Keiko. To determine the answer, we need to look at the options given: three weeks, three months, three years, or three days. Based on the options, the correct answer would be three days, as it is the smallest unit of time mentioned indicating the shortest period of time since Henry received a letter from Keiko.