Explanation

Gregor Mendel conducted his classic hybridisation experiments on the garden pea (Pisum sativum) between 1856 and 1863, studying traits such as seed shape, flower colour and plant height to formulate the laws of inheritance. The fruit fly (Drosophila) was used later by T.H. Morgan, the four o'clock plant (Mirabilis jalapa) by Correns for incomplete dominance, and groundnut was not Mendel's organism. Hence the garden pea is correct.