Go through the following points.
I. Dicliny (unisexuality of flower)
II. Dichogamy (protoandry or protogyny)
III.Self sterility /self incompatibility
IV. Heterostyly
V. Herkogamy (Barrier between anther and stigma of a bisexual homogamous flower)
The above contrivances prevent

Options

A. Geitonogamy
B. Xenogamy
C. Allogamy
D. Autogamy

Answer

D