You can see your cells using the scanning and the 10x objective lenses. when you add oil to your slide and move up to the oil immersion objective you can not focus on anything. your instructure comes over and wipes off your oil and turns over the slide and adds new oil and moves the oil immersion objective into place. you can now focus on your cells. knowing how an oil immersion objecttive work why couldnt you see the cells when the slide was upside down?

Generally, oil immersions are used when one has to witness the cells from the inside and is mostly seen at 100X objective. The oil immersions are used to cope with the RI or the refractive index, as when light goes from a substance of one RI to the substance of another RI the light gets scattered.  

One is not able to see the cells when the slide is placed upside down, however, when the slide was turned over then everything becomes clear. As basically when we look through the microscope then initially the top thread comes into focus succeeded by the middle and eventually the bottom one.  

However, when one put the slide upside down then at that time the slide function as the first thread and the cells function as the second thread. As the slide is formed of dense and thick substance it is tough to focus the second thread or the cells via it. This is the reason that one is not able to witness anything when the slide is kept in this position.  

b is the answer and if you need further with anything message me