Production possibility frontier approach:
The marginal opportunity cost of a good equals the slope of the production possibility frontier, the ratio of units of one good given up to the units of the other good gained: \( MOC = \left| \dfrac{\Delta Y_1}{\Delta Y_2} \right| \). Here resources are shifted so that \( \Delta Y_1 = -500 \) units and \( \Delta Y_2 = +500 \) units. Substituting gives \( MOC = \left| \dfrac{-500}{500} \right| = 1.00 \). Since exactly one unit of \( Y_1 \) is sacrificed for every unit of \( Y_2 \) gained, the marginal opportunity cost is 1, confirming option (3).