a. Agriculture
Pure competition involves an industry in which there is no product differentiation, low barriers to entry and exit in the industry, and there are many sellers, enough so that no individual seller has enough market share to influence the market price.
None of the other choices come close to this definition. Agriculture is not as good of an example as it used to be because farming is no longer dominated by family size farms, but by large corporations. Also, government protectionism and subsidies paid for specific agricultural products change the make-up of the industry. But of the possible choices, this is the correct one. Utilities and Major League Baseball are monopolies (MLB is a special form of monopoly involving different ownerships of different teams, but that's another story), and the airline industry is an oligopoly.
