The correct answer is A. Straight line.
A production possibility curve (PPC) is a graph showing the maximum combinations of two goods that can be produced in a given period of time, given the available factors of production and technology. The PPC is a straight line when the marginal (additional) opportunity cost is a constant. This means that the amount of one good that must be given up to produce an additional unit of the other good is always the same.
A convex PPC occurs when the marginal opportunity cost is increasing. This means that the amount of one good that must be given up to produce an additional unit of the other good is increasing as more of the other good is produced. A concave PPC occurs when the marginal opportunity cost is decreasing. This means that the amount of one good that must be given up to produce an additional unit of the other good is decreasing as more of the other good is produced.
A backward-bending PPC occurs when the marginal opportunity cost of producing one good becomes negative. This means that the more of one good that is produced, the less of the other good must be given up to produce it. This can occur when there are increasing returns to scale in the production of one good.
In conclusion, the PPC is a straight line when the marginal (additional) opportunity cost is a constant. This means that the amount of one good that must be given up to produce an additional unit of the other good is always the same.