I'd like to think your course has prepared you for some of this assignment but I'll help get you started. They do make some transformers with selectable primaries - the transformer is wound so that by selecting the right taps you can work with various input voltages. You'd need to select a transformer with the primary voltages you have in mind and with a secondary that is of adequate capacity.
Keep in mind that the transformer might not be the first component you select and that this might be an iterative process, especially if the requirement is to specify real world components. An example - you might first determine that a 28 vac secondary is sufficient but find the only transformer made is 32 vac. That might put a greater demand on the regulator, pass transistor, heat sink, etc.
Note also that there are quite a few books out there on power supply design. I know that a number of my ARRL amateur radio handbooks provide lots of good info though it's fragmented requiring that you think thru the whole process.