Answer:
Biased samples are those samples wherein during the survey some members of the whole population are less likely considered for survey.
The correct answers are written in bold.
1. Surveying the boys in the freshman class - This is biased and not random as only boys are included in the survey, not the girls.
2. Surveying every other student in the cafeteria - This is non biased as it is random and every other student can be anyone.
3. Surveying all the students in her math class - This is biased and not random as it says all the students are included. Random students are not selected.
4. Surveying 20 students from each grade at random - This is a random selection.
5. Surveying the students in the chess club - This is also biased and not random as chess club can have few students as compared to hundreds of students from the school.