probabilities

A random selection of files from a student counseling center revealed the following reasons why college students seek services:

1. What does it mean to say the files were randomly selected?
2. How would the researcher randomly select files? In other words, what does the process look like?
3. What is the probability that if we pulled another student file from the counseling center the student would fall in each of the following categories a) mental health issues, b) learning/school issues OR relationship issues, c) any category except other?
4. Would our probabilities and results be different if we used convenience sampling? Why or why not?