After observing the variations Hugo De Vries first of all described the mutation in

The correct option is A Oenothera lamarckiana
Hugo de Vries studied inheritance of flower colour in Oenothera lamarckiana (evening primrose) and proposed mutation theory that says that mutations are sudden, random, and inheritable and discontinuous variations which serve as raw material for evolution.

George Beadle and Edward Tatum gave molecular definition of a gene while working on nutritional mutants in Neurospora crassa and proposed the one gene–one enzyme hypothesis.

Mendel conducted hybridization experiments on garden pea plant, Pisum sativum and proposed “Laws of inheritance” but did not observe any mutations.

Morgan and Muller identified radioactive nature of X-rays to induce point mutations while working on sex linked inheritance in Drosophila.

So, the correct answer is option A.