If you were scanning a slide to find a particular object, which objective would be better to use?

You always start with the lowest objective to scan for your organism. This is because as you use higher and higher objectives, your field of view gets really small. Think about it: if you were looking at at an "e" on the microscope and you used the 10x or 40x you may land in-between the lines of the loop of the "e". If you started on the lowest, you can see the whole thing and then decide where you need to look in more detail.