Final answer:
Decreasing Earth's rotational period by one hour would most significantly shorten the time period between high tides for a stationary observer on Earth due to the faster rotation bringing an observer through tidal bulges more rapidly.
Step-by-step explanation:
The change that would most significantly shorten the time period between high tides seen by a stationary observer on Earth is decreasing Earth's rotational period by one hour (option d). The formation of tides on Earth is a result of the gravitational forces between the Earth and the Moon, and as the Earth rotates, people experience the high and low tides.
Since the actual tidal period is approximately 12 hours and 25.2 minutes, reducing the rotational period of the Earth by an hour would mean that an observer would be carried through the tidal bulges more quickly, thus experiencing high tides more often within a 24-hour cycle.
Options involving changes to the orbital period of the Moon or Earth would not have as immediate an effect on the frequency of tides as the rotation of the Earth.