If the demand curve for cooking oil shifts from D1 to D2, one could say that . . .

The question asks us to consider the scenario where the demand curve for cooking oil shifts from $D_1$ to $D_2$.
Looking at the diagram, the curve $D_2$ is to the right of $D_1$. A rightward shift of the demand curve indicates an increase in demand.

• a) The quantity demanded of cooking oil has decreased to Q1 and price has fallen to P2.

  • A shift from $D_1$ to $D_2$ represents an increase in demand. If supply remains constant (e.g., at $S_1$), the equilibrium quantity would increase (from $Q_1$ to $Q_2$) and the equilibrium price would increase (from $P_1$ to $P_2$). This statement describes a decrease in quantity and a fall in price, which contradicts an increase in demand. Therefore, this option is incorrect.

• b) The price of coconut oil which is a substitute for cooking oil must have fallen.

  • If the price of a substitute good (like coconut oil) falls, consumers would buy more of the substitute and less of cooking oil. This would lead to a decrease in demand for cooking oil, causing the demand curve to shift to the left. The diagram shows a rightward shift ($D_1$ to $D_2$), which is an increase in demand. Therefore, this option is incorrect.

• c) There has been an increase in demand for cooking oil.

  • As established in Step 1, a rightward shift of the demand curve from $D_1$ to $D_2$ signifies an increase in demand. This statement accurately describes the shift shown in the diagram.

• d) The higher price of cooking oil has caused the quantity demanded to fall from OQ1 to OQ2.

  • A change in the price of the good itself causes a movement along the demand curve, not a shift of the entire curve. A higher price would cause a decrease in the quantity demanded (moving up along a single demand curve). The question describes a shift of the demand curve. Furthermore, if demand shifts from $D_1$ to $D_2$ (with supply $S_1$), the quantity increases from $Q_1$ to $Q_2$, not falls. Therefore, this option is incorrect.

Based on the analysis, the shift from $D_1$ to $D_2$ represents an increase in demand for cooking oil.

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