An email arrives informing Rick that there is a limited time to act to get a software package for free and that the first 50 downloads will not have to be paid for. What social engineering principle is being used against him?

Attackers are attempting to influence Rick with a combination of scarcity and urgency. Thus, in this case, you should answer scarcity since urgency is not listed. In many social engineering principle questions, more than one of the principles may be in play, and you will need to answer with the principle that is correct or more correct for the question. In this case, there is no intimidation or claim to authority, and consensus would require some form of validation from others.