The short run marginal cost (SMC), average variable cost (AVC) and short run average cost (SAC) curves are all u shaped.

The reason behind it is the law of variable proportion, which states that in initial stages of production in short run the average and marginal cost fall.

Subsequently with constant returns to labour the cost curve become constant and reach their minimum point, which represents the optimum combination of capital and Labour. Beyond this with every additional unit of labour will increase the cost and as the marginal product of labour starts falling the cost curve starts rising due to decrease in returns to labour.

Let's understand it with help of graph –

Cost Schedule

GRAPH