Plant of Vallisneria is

Vallisneria is a submerged hydrophyte. It is a dioecious plant with the flowers borne under water. The male flowers detach on maturity and float on the surface of the water while the female flowers grow under water and they come up to the surface with the help of their thin long stalk. As the stalk grows further, the flowers which were vertical become horizontal and thus, a cup like depression is formed around the female flower. The male flowers which are floating on the surface of the water, fall into the cup and due to jerk, the anthers dehisce and pollen is transferred to the stigma and thus, pollination occurs on the surface of the water. This kind of pollination found in Vallisneria is called ephydrophily. 

So, the correct answer is B.