According Lamarckism long necked Giraffes evolved because :

The correct option is B Of stretching of necks by short one never many generations.
The ancestors of giraffe had short neck and shorter fore limbs. Due to change in the environmental condition they were forced to feed on taller trees. In order to reach to the food they had to stretch their neck and lift their fore limbs from the ground. This effort they made through out the life ,due to which their forelimbs and neck became slightly longer.

So, the correct answer is 'Of stretching of necks by short one never many generations'