The correct answer is A. OPEX.
Consumer surplus is the difference between the maximum amount a consumer is willing to pay for a good and the price they actually pay. In the diagram, the maximum amount the consumer is willing to pay is represented by the demand curve, $D$. The price the consumer actually pays is represented by the equilibrium price, $P$. The area between the demand curve and the price line, $OPAE$, is the consumer surplus.
Option B, ODEX, is the producer surplus. Producer surplus is the difference between the minimum price a producer is willing to accept for a good and the price they actually receive. In the diagram, the minimum price the producer is willing to accept is represented by the supply curve, $S$. The price the producer actually receives is represented by the equilibrium price, $P$. The area between the supply curve and the price line, $BDE$, is the producer surplus.
Option C, PDE, is the total surplus. Total surplus is the sum of consumer surplus and producer surplus. In the diagram, the total surplus is represented by the area between the demand curve and the supply curve, $OPAE+BDE$.
Option D, None of these, is incorrect.