Final answer:
The point estimate for the average stay time of students during online lectures is the sample mean, which is 85 minutes.
Step-by-step explanation:
The point estimate of a parameter is the best single value that represents the parameter of interest. In this case, the professor has conducted a study to estimate the average stay time of students during two-hour online lectures. From the sample, the mean is found to be 85 minutes, and this mean is the point estimate for the population mean stay time.