The correct answer is B. linear.
An isoquant is a curve that shows all combinations of two inputs that produce the same level of output. If there is perfect substitution between two factors of production, then the isoquant will be a straight line. This is because the firm can use any combination of the two inputs to produce the same level of output, and it will not matter which combination it uses.
Option A is incorrect because a non-linear isoquant would indicate that there is some degree of complementarity between the two factors of production. This means that the firm would not be able to use any combination of the two inputs to produce the same level of output, and it would matter which combination it uses.
Option C is incorrect because a right-angled isoquant would indicate that there is no substitution between the two factors of production. This means that the firm would have to use a certain amount of each input in order to produce any level of output.
Option D is incorrect because a positively sloped isoquant would indicate that there is a negative relationship between the two factors of production. This means that the firm would have to use less of one input in order to use more of the other input.