Based on the information provided, the most reasonable conclusion is:
A boiling point at 1.5 atmospheres could be predicted fairly well.
This conclusion is justified because the results indicate a nearly constant increase in boiling point with increasing pressure within the tested range (0.5 to 1.4 atmospheres). Therefore, it is reasonable to extrapolate that this trend would continue slightly beyond the maximum tested pressure (1.4 atmospheres) to predict the boiling point at 1.5 atmospheres. However, predicting at 15 atmospheres would be less certain due to the lack of data in that range.