Self-pollination is fully ensured if

The correct option is "the flower is cleistogamous."

To understand why this is the correct answer, let us break down the concepts of pollination and the conditions that influence it:

1. What is Cleistogamy?
Cleistogamy is a condition in which flowers do not open at all. Since the flowers remain permanently closed, the mature anthers and stigma are kept in close proximity inside the bud. Because no external pollen can enter the closed flower, and the pollen from the flower's own anthers cannot escape, self-pollination (specifically autogamy) is fully and absolutely ensured.

2. Why other options do not fully ensure self-pollination:
• Bisexual flower: Although a bisexual flower contains both male (stamens) and female (pistil) reproductive organs, it does not guarantee self-pollination if the flower opens. Open bisexual flowers (chasmogamous flowers) are still exposed to cross-pollination by wind, water, or animal pollinators.
• Style longer than the filament: If the style is significantly longer than the filament (a condition known as heterostyly), the pollen grains from the shorter stamens are less likely to fall onto the elevated stigma of the same flower. This is actually an evolutionary adaptation to prevent self-pollination and encourage cross-pollination.
• Different maturity times: When the anther and pistil mature at different times (a condition known as dichogamy, which includes protandry and protogyny), it prevents self-pollination because when the pollen is shed, the stigma of the same flower is not receptive, or vice versa.

Therefore, cleistogamy is the only condition among the choices that completely prevents cross-pollination and fully ensures self-pollination.