In which one of the following, both autogamy and geitonogamy are prevented?
(1) Maize
(2) Wheat
(3) Papaya
(4) Castor

Option (3) Papaya is correct.

This is because –  Papaya is a dioecious plant in which male and female flowers are produced on two separate plants. Hence it prevents both autogamy as well as geitonogamy.
Castor and Maize : Monoecious and unisexual
Wheat : Monoecious and bisexual