In which one of the following, pollination is autogamous?
A. Geitonogamy
B. Xenogamy
C. Chasmogamy
D. Cleistogamy

Hint: There are two types of pollination i.e., self pollination and cross pollination and autogamy is a type of self pollination. Few flowers remain close in order to avoid cross pollination and that type of pollination is autogamous.

Complete answer:
First of all, you should know what autogamy is. Autogamy is a type of self pollination in which the pollen grains are transferred from another to stigma of the same flower and now we need to find which pollination is autogamous from the given options.
If we look at options now:
>Option A is Geitonogamy. Geitonogamy is a type of self pollination where pollen grains are transferred from anther of one flower to stigma of another flower on the same plant but still Geitonogamy pollination is not autogamous as this is self pollination between genetically similar but physically different flowers and that is not autogamy. Therefore, this option is incorrect one.
>Moving to option B i.e., Xenogamy. Xenogamy is not a type of self pollination but it is a type of cross pollination where the pollen grains are transferred from anther to stigma but on different plant of same species and this makes this option incorrect as in the question pollination which is autogamous is asked and it is a type of self pollination. Hence, this option gets eliminated.
>Moving to option C i.e., Chasmogamy. It is a type of cross pollination where flowers that open at maturity facilitate exposure of pollen and stigma and these flowers are referred to as chasmogamous flowers which are large and colourful flowers having scent to attract pollinators. It is not autogamous as it is not a type of self pollination. Therefore, this option is incorrect.
>Last option D is Cleistogamy. Cleistogamous flowers avoid cross pollination as they remain close to avoid it and these flowers exhibit different modification in order to facilitate self pollination and thus this is the correct option as they do not open at all and only autogamy occurs in these flowers.
Thus, the correct answer is option D i.e., Cleistogamy.

Note: Autogamy refers to self pollination where there is transfer of pollen grains from anther to stigma and cleistogamy pollination is autogamous as the cleistogamous flowers remain close to avoid the cross pollination and only autogamy occurs.