The correct answer is D. All of these.
AVC (average variable cost) is U-shaped because as the firm produces more output, its variable costs per unit decrease due to economies of scale. However, as the firm produces more output, its variable costs per unit eventually increase due to diseconomies of scale.
AFC (average fixed cost) is always decreasing because fixed costs are spread out over more units of output as the firm produces more.
MC (marginal cost) is U-shaped because as the firm produces more output, its marginal costs decrease due to increasing returns to scale. However, as the firm produces more output, its marginal costs eventually increase due to diminishing returns to scale.