The best example of a perfectly competitive market among the options given is d) Farming.
In a perfectly competitive market, there are many buyers and sellers, and no single entity can influence the market price. The products offered are homogeneous, meaning they are perfect substitutes for one another. In the case of farming, many farmers produce similar crops, and individual farmers cannot set the prices; they must accept the market price determined by overall supply and demand.
On the other hand, diamonds, athletic shoes, and soft drinks all have characteristics that diverge from perfect competition. Diamonds, for instance, are differentiated products and require significant branding efforts. Athletic shoes have brand loyalty and differentiation, allowing companies to set their prices above marginal costs. Soft drinks face brand loyalty and aggressive marketing, which also disrupts the conditions of a perfectly competitive market. Therefore, farming with its many producers and standardized products stands out as the clearest example of perfect competition.