Which is a unisexual flower?

Think it through first, then pick an option.

Answer and explanation

Correct answer: a Ridge gourd

The ridge gourd is a unisexual flower like bottle gourd and pumpkin; one of the two essential whorls is missing.

Why the other options are wrong

b Hibiscus: The hibiscus flower is bisexual, as it has both androecium and gynoecium.
c Pea: The pea is a bisexual flower; it has both essential whorls.
d Rice: Rice flowers carry both stamens and carpels, so they are bisexual.