Tetradynamous stamens are found in

According to the relative size of different stamens in a flower, they are grouped as tetradynamous, didynamous and heterostamonous. Presence of six stamens, two shorter in outer whorl and four longer in inner whorl, is termed as tetradynamous. It is characteristic feature of crucifers, for example Brassica.
Pisum is characterized by presence of nine stamen filaments united into a sheath that surrounds the pistil and one separate stamen filament, a condition referred to as diadelphous.
In Cucurbita, stamens are of same size and are completely fused through filament as well as anthers and are termed as synandrous.
In Hibiscus, base of filament of same size stamen is attached to stem and forms a staminal column/staminal tube. 
Therefore, the correct answer is option A.