Solution

Onion has bulb as it is the modified stem, its aestivation is imbricate ( if the margins of sepal and petals overlap one another but not in any particular direction), placentation is axile (the ovary is sectioned by radial spokes with placentas in separate locules).
Tomato has valvate aestivation (margins of adjacent petals or sepals touch each other without overlapping), placentation is axile (the ovary is sectioned by radial spokes with placentas in separate locules), fruit is berry or capsule.
In maize, the dark reaction is $$C_4$$ pathway, the vascular bundle is open, in seed scutellum ( modified seed leaf) is present.
In pea plant the dark reaction is $$C_3$$ pathway, the seed is non endospermic, aestivation is valvate or imbricate.