Cleistogamous flowers are found in:
A. Arachis hypogaea
B. Solanum tuberosum
C. Cucumis melo
D. Allium cepa

Hint: Cleistogamous flowers are those flowers which remain closed throughout their lifetime. Cleistogamy is the type of self-pollination which takes place in such closed flowers. Since these flowers do not open at all hence, there is no scope for cross pollination.

Complete answer:
To solve the given question, we must know about the cleistogamous flowers. These flowers are those flowers which do not open due to which their reproductive whorls are not exposed. The type of pollination which takes place inside them is autogamy where the transfer of pollen grains takes place from the anther to the stigma of the same flower.
The anthers dehisce inside the closed flowers and the pollen grains from the dehisced anthers fall on the stigma of the same flower. These pollens fuse with the egg cells to give rise to the zygote which grows further to form a complete plant.
Cleistogamous flowers are Viola mirabilis and Oxalis acetosella. In plants like Commelina benghalensis both chasmogamous and cleistogamous flowers are present.
The pollinators of Allium cepa and Cucumis melo are bees and the pollination in them is entomophily. The two of them i.e. Onion and Pumpkin produce bright flowers and so bees get attracted to them and bring about pollination.

Cleistogamous flowers are found in Arachis hypogaea. Therefore the correct answer to the question is Arachis hypogaea.

Note: Here it must be noted that cleistogamous flowers are different from chasmogamous flowers. These two must not be confused with each other. While cleistogamous flowers do not open at all, chasmogamous flowers open when they reach maturity. Cleistogamous flowers are only self-pollinated. On the other hand, chasmogamous flowers are usually cross-pollinated and rarely self-pollinated.