The correct answer is A. a smooth curve.
A Leontief isoquant is a smooth curve because it represents the combinations of two inputs that produce the same level of output. The curve is smooth because there is no substitution between the two inputs. In other words, if you want to produce more output, you need to use more of both inputs.
Option B is incorrect because a line with no kinks would represent a situation where there is perfect substitution between the two inputs. This is not the case in a Leontief production function, where the two inputs are complementary.
Option C is incorrect because a line with many kinks would represent a situation where there is imperfect substitution between the two inputs. This is not the case in a Leontief production function, where the two inputs are complementary.
Option D is incorrect because a line parallel to the axis with a kink would represent a situation where one input is completely useless. This is not the case in a Leontief production function, where both inputs are necessary to produce output.