Final answer:
The actual melting point of a solid is a range, from when the substance begins to melt to when it is completely melted, which for this sample is 138-141.5°C. The midpoint of this range, 139.75°C, could represent the most accurate single melting point if required.
Step-by-step explanation:
If a solid sample melted at 138-141.5°C, the actual melting point is the temperature range over which the substance transitions from a solid to a liquid. It is not a single temperature, but rather a range that can indicate the purity of the substance. For a pure substance, the melting point should be relatively sharp, occurring over a small temperature range. However, impurities can cause a broader range, as observed in the given data.
The universally accepted way of reporting a melting point is to provide this range, which shows where the solid first began to melt (the initial melting point) until the point where the solid has completely melted (the final melting point). Therefore, the given range from 138°C to 141.5°C represents the actual melting point of the sample. If one must choose the most representative single temperature for the melting point, it would be the midpoint of the range. Hence, the answer would be (d) 139.75°C.