In the short run, the shape of marginal cost, average variable cost and short run average cost curves are:

Option 1 -> Concave curves bend inward, which does not represent the typical shape of short-run cost curves.

Option 2 -> These curves are not continuously downward sloping; they initially decline but eventually rise.

Option 3 -> U-shaped curves first decline, reach a minimum point, and then rise due to the law of diminishing marginal returns.

Option 4 -> Convex curves bend outward, which does not accurately describe these cost curves.

Hence, Option 3: U-shaped -> In the short run, the Marginal Cost (MC), Average Variable Cost (AVC), and Short-run Average Cost (SAC) curves all exhibit a U-shape. This occurs because initially, as output increases, fixed costs are spread over more units and variable inputs are used more efficiently, causing average costs to decline. However, after reaching an optimal production level, the law of diminishing marginal returns sets in - adding more variable inputs to fixed factors leads to less productive use of resources, causing costs per unit to rise. The MC curve intersects both AVC and SAC at their minimum points, which is a key characteristic of this relationship. -> correct