I think there is no question...
A sampling frame would be how you could identify college students. That is what the source would be. A selection bias is something that interferes with your ability to sample -what would that be in this case.
An example might be an important part of your sampling frame might refuse to answer samples. Or be unable to. For instance, phone sampling has a problem sampling young people who (in days of yore anyhow) not have phones. Or might not have a phone number you could get at.
Yeah, most forms of selection bias fall under collider bias, meaning you are selecting on a variable that is the effect of the exposure and outcome. For example, being young effects phone status, and the outcome effects phone status (say they are very sick thus unemployed, so no phone). So whether or not being young is associated with the outcome a backdoor path connects them biasing the estimate since you are controlling for an effect of both the IV and DV variables via your selection process.