Convert `:`

Explanation

`a.`
`b.` It is Diels `-` Alder reaction and gives two stereoisomers exo `(` equilibrium or thermodynamically controlled `)` and endo `(` kinetically or rate controlled `)` products.

(exo and endo) Methyl bicylo `[2.2.1]` hept-5-ene-2-carboxylate (or methanoate)
c. `O_(2)` is a diradical with the same spin, so it can form one bond with diene, forming an intermediat and does not form the second bond because it has two electrons with the same spins (triplet state)