The unit rate is when the change in x is 1, so basically in this case the x coordinate is 1. That applies to point C. The answer is not point B as B is not on the diagonal line.
Unit rate = slope = rise/run = (change in y)/(change in x)
If every £3 represents 2 of another currency, you just need to figure out how many groups of £3 are in £100 (so £100/£3 = 33.33). Now multiply this value by 2 to get the equivalent of £100 in another currency.