The correct answer is: A. Downward to the left.
An isoquant is a curve that shows all combinations of inputs that produce the same level of output. It is downward-sloping because as we use more of one input, we must use less of the other input in order to keep output constant. This is because inputs are often complementary, meaning that they work together to produce output. For example, if we are producing bread, we need both flour and water. If we use more flour, we can use less water, and vice versa.
Option B is incorrect because an isoquant slopes downward to the left, not to the right.
Option C is incorrect because an isoquant slopes downward to the left, not upward to the left.
Option D is incorrect because an isoquant slopes downward to the left, not upward to the right.