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 mechanisms that ensure self-pollination step-by-step:

1. Understanding Self-Pollination:
Self-pollination is the transfer of pollen grains from the anther (male reproductive part) to the stigma (female reproductive part) of the same flower or another flower on the same plant. For self-pollination to be fully ensured, there must be no possibility of pollen from another flower (cross-pollination) reaching the stigma.

2. What is Cleistogamy?
Cleistogamy is a condition where flowers never open. Since these flowers remain closed, the reproductive organs (anthers and stigma) are completely isolated from the external environment. Therefore, pollen grains from other flowers cannot enter, and the pollen from the flower's own anther is forced to land on its own stigma.

3. Why other options do not fully ensure self-pollination:
- Bisexual flowers: Although bisexual flowers have both male and female reproductive organs, they can still open (chasmogamous flowers), which allows cross-pollination by wind, water, or insects. Therefore, self-pollination is not fully ensured.
- Style longer than filament: If the style (which holds the stigma) is longer than the filament (which holds the anther), it physically separates the stigma from the pollen of the same flower. This promotes cross-pollination rather than self-pollination.
- Different maturity times: If the pistil and anther mature at different times (dichogamy), self-pollination is prevented because when pollen is released, the stigma is not ready to receive it, or vice versa.

Thus, cleistogamy is the only mechanism that absolutely guarantees and fully ensures self-pollination.