The correct option is C Cucumber
A flower that only has stamen or carpels is said to be "unisexual". The flowers of the cucumber plant are unisexual. Flowers of onion, pea and china rose plants are bisexual.
Unisexual flowers which contain either androecium or the gynoecium are found in cucumber whereas in the other options given the flowers are bisexual.
The correct solution is option C.