Diadelphous stamens are found in

Concept:

• Stamens are the male reproductive organs of flowering plants.
• Stamens consist of an anther, the site of pollen development, and in most species a stalk-like filament, which transmits water and nutrients to the anther.

• Stamens in a flower may either remain free (polyandrous) or may be united in varying degrees.
• When the stamens are united into one bunch or one bundle are called monadelphous, and when united into two bundles are called diadelphous.

​Explanation: 

•  In china rose, the stamens are united into one bunch or one bundle (monadelphous).
• In Citrus the stamens are united into more than two bundles (polyadelphous). 
• In peas, the stamen are united into two bundles (diadelphous).

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Therefore, the correct answer is option 4, Pea.