(d) Explain the effect of the different SA:Vol ratios on the rate of osmosis into the potato.

Best way to explain would be to draw a cell with both a big and a large SA:Vol ratio. Visually get to see the effect, to understand why the rate of osmosis changes.

note: SA = surface area and Vol = volume

essentially means how large the SA is in comparison to the volume 

Osmosis = diffusion of a liquid from a high to low concentration gradient.

The larger the  SA:Vol ratio, the greater the rate of osmosis into the potato.