The correct answer is: A. perfect substitutes.
An isoquant curve is a graph that shows all the combinations of inputs that can produce the same level of output. If inputs are perfect substitutes, then any combination of inputs on the isoquant curve will produce the same level of output. This is because perfect substitutes are inputs that can be used in any proportion to produce the same output. For example, if labor and capital are perfect substitutes, then a firm could produce the same level of output by using 100 workers and 1 machine, or by using 50 workers and 2 machines, or by using 25 workers and 4 machines, and so on.
The other options are incorrect because they do not describe the relationship between inputs and output when inputs are perfect substitutes. Option B, good substitutes, is incorrect because good substitutes are inputs that can be used in different proportions to produce the same output, but there is a limit to how much one input can be substituted for another. For example, if labor and capital are good substitutes, then a firm could produce the same level of output by using 100 workers and 1 machine, or by using 50 workers and 2 machines, but it would not be able to produce the same level of output by using 1 worker and 100 machines. Option C, poor substitutes, is incorrect because poor substitutes are inputs that can be used in different proportions to produce the same output, but there is a very limited range of proportions in which they can be used. For example, if labor and capital are poor substitutes, then a firm could produce the same level of output by using 100 workers and 1 machine, or by using 50 workers and 2 machines, but it would not be able to produce the same level of output by using 1 worker and 10 machines, or by using 100 workers and 10 machines. Option D, used in a fixed ratio, is incorrect because inputs that are used in a fixed ratio are not substitutes. For example, if labor and capital are used in a fixed ratio of 1:1, then the firm cannot produce the same level of output by using 100 workers and 1 machine, or by using 50 workers and 2 machines.