An isoquant is a graphical representation of all the possible combinations of inputs that can produce a certain level of output. It is similar to an indifference curve in consumer theory, but instead of representing combinations of goods that provide the same level of satisfaction, an isoquant represents combinations of inputs that result in the same level of output.

a) An isoquant that is further from the origin represents greater output:
An isoquant is usually drawn as a curve, and the distance of the curve from the origin represents the level of output. As we move away from the origin along the isoquant curve, we are able to produce higher levels of output. This is because the combinations of inputs represented by points further from the origin are more productive and efficient in generating output. Therefore, option 'A' is correct.

b) A flatter isoquant represents the trade-offs between inputs that are poor substitutes:
The slope of an isoquant represents the rate at which one input can be substituted for another while keeping the level of output constant. If the isoquant is flatter, it means that the inputs are poor substitutes for each other. In other words, a small change in one input can be offset by a larger change in the other input without affecting the level of output. This implies that the firm can easily adjust its input mix without significantly impacting its production. Therefore, option 'B' is incorrect.

c) A negatively sloped isoquant represents input combinations associated with Stage I of production:
The slope of an isoquant is negative because the inputs are assumed to be imperfect substitutes for each other. As a result, as the firm increases the quantity of one input, it needs to decrease the quantity of the other input to maintain the same level of output. This negative slope of the isoquant is associated with the initial stages of production, where increasing the quantity of one input has a significant positive impact on output. Therefore, option 'C' is incorrect.

d) All of the above are correct:
Based on the explanations provided above, it is clear that option 'A' is correct, but options 'B' and 'C' are incorrect. Therefore, option 'D' is incorrect as well.

In conclusion, the correct answer is option 'A'.