Answer:
b.) To estimate the mean number of classes that students take at his university, Samuel collects data from a randomly selected proportionate number of students from each grade level.
c.) David wants to estimate the mean grade point average of students at his school. He collects data by recording the grade point average of every 25th student on the list of students after a randomly selected first student.
d.) Helen wants to estimate the male to female ratio of the residents of her city. She collects data by recording the sex of every 50th resident after selecting a random starting point on a list of residents.
Why? Because a sample is only biased if some individuals of the population are more or less likely to be chosen than others.
The sample from choice B isn't biased because every student has the same opportunity of being selected. It's the same predicament with choice C; it's also non-biased because again, each student has a chance of being in it, there is no preference.
And finally, the sample from option D is non-biased, because every resident that is being selected has an equally fair chance of being selected at random.
Hope this helps ;)