Which of the following elements forms a water-soluble, alkaline oxide? An element burns in oxygen to form a solid oxide which dissolves in water. The pH of the resulting solution is greater than seven. Which one of the following could the element be?

A. sulphur

B. phosphorus

C. magnesium

D. sodium

E. carbon

The right answer is sodium. But I don't really understand why it can't be magnesium as well.
solid oxide which dissolves in water


Magnesium oxide $\ce{MgO}$ is (practically) insoluble in water.

You might be interested to know that burning of magnesium was the classical flash light in the early days of photography:  Wikipedia.