The best option to examine the hypothesis that over-watering can lead to plant death is:
b. Over-water a group of plants and compare it to an equal group of plants that are watered normally.
This approach allows for a controlled comparison between two groups of plants (over-watered vs. normally watered), making it possible to draw conclusions about the effects of over-watering on plant health. It also establishes a clear experimental design with a control group, which is essential for valid scientific experimentation.