Final answer:
The best hypothesis for examining the effects of salt water on plant growth is A) 'If salt water is used to water plants, then the plants will not grow as tall as those watered with tap water.' This provides a clear and testable hypothesis that is crucial for a scientifically valid experiment.
Step-by-step explanation:
The best hypothesis for an experiment investigating the impact of salt water on the growth of plants would be A) If salt water is used to water plants, then the plants will not grow as tall as those watered with tap water.
This hypothesis is testable, specific, and sets up a clear relationship between the independent variable (salt water) and the dependent variable (plant growth), which is a fundamental component of a scientifically valid hypothesis.
Regarding the student's statement about a researcher growing plants in 10 mM NaCl solutions, the question to be posed could involve investigating the response of plants to varying levels of salinity by growing the plants in a series of lower-salinity conditions to observe any potential differences in growth.
For example: 'How does gradually decreasing salinity from 10 mM NaCl affect the growth and health of the same varieties of plants?'