Water --- into ice when it is cold enough.

D.can be changed
情态动词 can 可以表示 可能性
本题用 must不妥 因为 must一般不能 对将要发生的动作 做出判断
其实 本题 可以用一般现在时 或一般将来时 或 can
Water _turns__ into ice when it is cold enough.
Water _will turn__ into ice when it is cold enough.
Water _can turn __ into ice when it is cold enough.