Understand the market structure.

In a perfectly competitive market, an individual farmer is one of many sellers producing an identical product. Because the farmer's output is a tiny fraction of the total market supply, they have no power to influence the market price. They are a price taker.

The market price for the farmer's product (e.g., wheat) is determined by the interaction of the total market demand and total market supply for that product.

In Graph 1, the market demand curve ($D_{market}$) intersects the market supply curve ($S_{market}$) at the equilibrium point, establishing the market price ($P_e$) and market quantity ($Q_e$).

Since the individual farmer is a price taker, they must accept the market price ($P_e$) determined in Graph 1. They can sell any quantity of their produce at this prevailing market price without affecting it. If they try to charge a higher price, they will sell nothing because buyers can get the identical product from other farmers at $P_e$. If they charge less, they would be leaving money on the table, as they can sell all their output at $P_e$. Therefore, the demand curve for an individual farmer is perfectly elastic at the market price.

In Graph 2, the individual farmer faces a horizontal demand curve ($d_{farmer}$) at the market price $P_e$. This means the farmer can sell any quantity (q) at this price. This horizontal demand curve also represents the farmer's Average Revenue (AR) and Marginal Revenue (MR).