$\begin{array}{1 1} \text{a. X constitutes the shallow water tanks and is called littoral zone} \\ \text{b. Y constitutes the open water zone upto which light can penetrate and is called profundal zone} \\ \text{c. Z constitutes the deep water zone where light cannot penetrate and is called limnetic zone} \\ \text{d. All of these} \end{array} $