The correct answer is A. A convex curve.
A production possibility curve (PPC) is a graph showing the maximum combinations of two goods that can be produced in a given time period, given the available factors of production and technology. The PPC is always downward-sloping, because to produce more of one good, the country must produce less of the other good.
A convex curve is a curve that bulges outward from the origin. This means that the opportunity cost of producing more of one good increases as the country produces more of that good. This is because as the country produces more of one good, it must use resources that are also used to produce the other good. As a result, the country has less resources available to produce the other good, and the opportunity cost of producing more of the first good increases.
A concave curve is a curve that curves inward towards the origin. This means that the opportunity cost of producing more of one good decreases as the country produces more of that good. This is because as the country produces more of one good, it can use less productive resources to produce the other good. As a result, the country has more resources available to produce the other good, and the opportunity cost of producing more of the first good decreases.
A straight line curve is a curve that has a constant slope. This means that the opportunity cost of producing more of one good is always the same. This is not realistic, because the opportunity cost of producing more of one good usually increases as the country produces more of that good.
A rectangular hyperbola is a curve that has two branches that meet at a point. This means that the country can produce any combination of the two goods on the curve, but it cannot produce any combination of the two goods that is not on the curve. This is not realistic, because the country can always produce some combination of the two goods, even if it is not on the PPC.