The correct answer is option 'C', which is the Long run average cost curve.

Explanation:
The Envelope Curve, also known as the Long run average cost curve, represents the lowest attainable average cost for producing different levels of output in the long run. It is derived from the combination of various short-run average cost curves.

The Envelope Curve is derived by connecting the points of tangency between the different short-run average cost curves and the long-run isocost line. The tangency points represent the most efficient combination of inputs for a given level of output.

- Definition of the Envelope Curve:

The Envelope Curve represents the long-run average cost curve, which shows the lowest average cost achievable for producing different levels of output in the long run when all inputs can be adjusted.

- Relationship between Short-run and Long-run Average Cost Curves:

The short-run average cost curve represents the cost of production when at least one input is fixed, while the long-run average cost curve represents the cost of production when all inputs can be adjusted.

The Envelope Curve is derived from the combination of various short-run average cost curves. Each short-run average cost curve represents a specific level of input combination and is tangent to the long-run isocost line at the point of minimum average cost.

- Determinants of the Envelope Curve:

The shape and position of the Envelope Curve are determined by various factors, including economies of scale, diseconomies of scale, and constant returns to scale.

- Economies of Scale: When an increase in the scale of production leads to a decrease in average cost, the Envelope Curve will be downward sloping.

- Diseconomies of Scale: When an increase in the scale of production leads to an increase in average cost, the Envelope Curve will be upward sloping.

- Constant Returns to Scale: When an increase in the scale of production does not affect average cost, the Envelope Curve will be a horizontal line.

- Uses of the Envelope Curve:

The Envelope Curve is used by firms to determine the optimal level of output and the most efficient combination of inputs for production. It helps firms identify the scale of production that minimizes average cost and maximizes profitability.

In summary, the Envelope Curve, or the Long run average cost curve, represents the lowest attainable average cost for producing different levels of output in the long run. It is derived from the combination of various short-run average cost curves and helps firms determine the optimal scale of production.