What is "what factor is most important in determining whether water is a solid?

The most important factor in determining whether water is a solid (ice) is Temperature. Specifically, water freezes and becomes ice at or below 0° Celsius (32° Fahrenheit) at standard atmospheric pressure. While other factors like Pressure can influence the freezing point to some degree, temperature is the dominant factor. For example, increased pressure can slightly lower the freezing point, but the change is usually negligible in everyday circumstances. Impurities dissolved in water can also affect the freezing point, typically lowering it. However, the overarching determinant remains temperature: below a certain temperature threshold, water transitions to its solid phase.