Assuming that the instant noodles are a representative of an inferior good, which of the following could explain a movement from point P1Q1 to point P2Q2 in the figure above?

The graph shows an initial equilibrium at point $(P_0, Q_0)$, where demand curve $D_0$ intersects supply curve $S_0$. The market moves to a new equilibrium at point $(P_1, Q_1)$, where demand curve $D_1$ intersects supply curve $S_1$.
From the graph:

• The equilibrium price has decreased ($P_1 < P_0$).
• The equilibrium quantity has increased ($Q_1 > Q_0$).
• The demand curve has shifted to the left ($D_0 \to D_1$), indicating a decrease in demand.
• The supply curve has shifted to the right ($S_0 \to S_1$), indicating an increase in supply.

The problem states that instant noodles are an inferior good. An inferior good is a good for which demand decreases as consumer income rises, and demand increases as consumer income falls.

We are looking for an event that could explain the observed decrease in demand and increase in supply.

• a) an increase in the price of a complement: An increase in the price of a complementary good would lead to a decrease in demand for instant noodles (leftward shift of the demand curve). This matches the observed demand shift. However, this option does not explain the increase in supply.

• b) an increase in buyers' income: Since instant noodles are an inferior good, an increase in buyers' income would cause a decrease in demand for instant noodles (leftward shift of the demand curve, $D_0 \to D_1$). This perfectly matches the observed demand shift. This option does not explain the increase in supply.

• c) a decrease in the buyers' income: For an inferior good, a decrease in buyers' income would lead to an increase in demand (rightward shift of the demand curve). This contradicts the observed leftward shift of the demand curve.

• d) an increase in the price of the good: A change in the price of the good itself causes a movement along the demand and supply curves, not a shift of the entire curves. This option is incorrect.

Both options (a) and (b) could explain the decrease in demand. However, the problem explicitly states that instant noodles are an inferior good. This specific characteristic directly links changes in income to changes in demand. Therefore, an increase in buyers' income is the most direct and relevant explanation for the observed decrease in demand for instant noodles. While the options do not provide a cause for the supply shift, option (b) correctly explains the demand component of the market movement based on the given information about the good.

The final answer is $\boxed{\text{b)}}$.