Based on the information provided, the conclusion that is reasonable is:
A boiling point at 1.5 atmospheres could be predicted fairly well.
This is inferred from the fact that the tests show a nearly constant increase in boiling point for each increase in pressure, meaning that the relationship between boiling point and pressure is likely linear or nearly linear within the tested range. Since 1.5 atmospheres is close to the range of pressures tested (0.5 to 1.4 atmospheres), it would be reasonable to predict the boiling point at that pressure with some confidence. In contrast, predicting boiling points at significantly higher pressures like 15 atmospheres would be less certain, as it is outside the range of the tested conditions.