What is opportunity sampling?
Here, you take advantage of a sample group which you have (eg you could use your S3 psychology class to represent all S3.
What is volunteer sampling?
This is when an experimenter advertises the experiment to try and get people to participate as participants.
What is random sampling?
This is where every member of the target population has a chance of being selected for the sample. It could be done by drawing names out of a hat or by using a computer program to randomly select names.