The correct answer is A. a straight line.
An isoquant is a curve that shows all the combinations of two inputs that will produce the same level of output. If two factors are perfect substitutes, then the isoquant will be a straight line. This is because the firm can use any combination of the two factors to produce the same level of output, and the marginal rate of substitution between the two factors will be constant.
A parabola is a curve that is symmetrical around its axis of symmetry. A rectangular hyperbola is a curve that has two branches that are symmetrical around its axis of symmetry. An L-shaped curve is a curve that has two straight lines that intersect at a right angle.
In the case of perfect substitutes, the firm can use any combination of the two factors to produce the same level of output. This means that the firm will always choose the combination of factors that minimizes the cost of production. The marginal rate of substitution between the two factors will be constant, because the firm will always be willing to trade one factor for the other at the same rate. This means that the isoquant will be a straight line.