Final answer:
The correct scientific conclusion is that water promotes better plant growth than milk, based on experimental observations. Spells and superstitions are not considered in scientific thinking. The focus of Gary's experiment was testing the effects of sunshine on plant growth.
Step-by-step explanation:
Based on scientific thinking, the conclusion that can be drawn is that water is the better liquid for tomato plant growth. This conclusion is derived from observing the experiment where the plant given water grew taller than the one given milk. Superstitions or spells, such as someone performing a spell or breaking a mirror, are not based on scientific evidence or thinking and therefore would not be considered valid scientific conclusions.
The subject of the question is Biology, as it involves understanding the growth conditions for plants, an area which falls under the biological sciences.
Regarding Gary's experiment, the variable being tested was the effect of sunshine on plant growth as opposed to the amount of water, the presence of detergent, or differences in the soil composition.