Hugo de Vries formulated the "Mutation Theory" based on the experiments he conducted on

The correct option is B Oenothera lamarckiana
Hugo de Vries, a Dutch Botanist formulated the Mutation theory. According to him, "new species are not formed by continuous variations but by the sudden appearance of variations", which he named as variations. He stated that mutations are heritable and persisted in successive generations.

He conducted his experiments on Oenothera lamarckiana (Evening Primrose) and found several aberrant types. These plants appeared to be new species, Hugo de Vries suggested from his experiments that new types of inherited characteristics may appear suddenly without any previous indication of their previous in the race. He believed that Mutation causes evolution and not the minor heritable variations which were mentioned by Darwin. Hence option D is correct.