Solution

Correct Option: D
Explanation:

Autogamy is a type of self-pollination wherein pollen grains from anther of flower are transferred to the stigma of same flower. Geitonogamy refers to pollination of pollen from anther of one flower to stigma of another flower, borne on the same plant, it is a type of self-pollination between genetically similar but physically different flowers. This makes option A incorrect.
Geitonogamy is type of cross-pollination wherein pollens from one plant are transferred to stigma of flower borne on different plant of same species. This makes option B incorrect.
Flowers that open at maturity to facilitate exposure of pollen and stigma for cross-pollination are referred to as chasmogamous flower, they are large, conspicuous colourful flowers with nectar/scent to attract pollinators. This makes option C incorrect.
Flowers that remain close to avoid cross-pollination and exhibit modifications to facilitate self-pollination are referred to as cleistogamous flowers. Thus, cleistogamous flowers are those that do not open at all and only autogamy occurs.
Therefore, the correct answer is option D.