The correct answer is: A. Monopoly.
A monopoly is a market structure in which there is only one seller of a good or service. This means that the monopolist has a very large degree of control over price fixation. The monopolist can set prices high without fear of losing customers to competitors, as there are no other firms selling the same good or service.
Perfect competition is a market structure in which there are many sellers of a good or service, and the products sold by these sellers are identical. This means that no one seller has any control over price, as buyers can easily switch to another seller if the price is too high.
Monopolistic competition is a market structure in which there are many sellers of a good or service, but the products sold by these sellers are not identical. This means that sellers have some control over price, but they cannot set prices too high or they will lose customers to other sellers.
Oligopoly is a market structure in which there are a few large sellers of a good or service. These sellers have a significant amount of control over price, as they can collude with each other to keep prices high.
In conclusion, the market structure in which a firm has a very large degree of control over price fixation is a monopoly.