The choices can be found elsewhere and as follows:
A.Acid(tartness)
B.Tartness(acid)
C.Tartness(lemon)
D.Lemon(acid)
I think the correct answer from the choices is option B. The tartness of a lemon (which depends on the concentration of citric acid) can be represented by Tartness(acid) or tartness as a function of acidity.
