It’s a Google Sheets feature called “Data Validation”. You’ll need a list somewhere of the items you want to show up in the drop-down (if there are only a few you can get away from that by including in the configuration screen), usually a new sheet would work for this. Then, in the column where you want the drop-downs to appear, select the cells where you want drop-downs, go to the “Data” menu, and choose “Data Validation”. The “Cell range” is where you want the drop-downs to appear, and the "Criteria is what you want to appear in the drop-down (I usually use range, but if only a few items you could choose list and type them in the box). Give it a try, it’s a great feature!