The correct option is D cleistogamy
Cleistogamous flowers do not open and reproductive parts are enclosed within the petals. Self pollination occurs in such flowers. Hence, cleistogamous flowers produce assured seed-set even in the absence of pollinators.
Chasmogamous flowers have open petals with exposed anthers and stigma. Herkogamous flowers have the stigma above the level of the anthers or vice versa. The pollen release and stigma receptivity in dichogamous flowers occur at different times.
Both herkogamy and dichogamy are mechanisms by which flowers prevent autogamy. These flowers undergo geitonogamy or xenogamy and therefore, require pollinators.