The correct answer is A. decrease.
The supply function is a curve that shows the relationship between the price of a good and the quantity of that good that firms are willing and able to supply. In a perfectly competitive market, firms are price-takers, meaning that they have no control over the price of their product. As a result, the only way for firms to increase their profits is to increase the quantity of output they produce.
The marginal cost (MC) of production is the additional cost incurred by a firm when it produces one more unit of output. When MC decreases, firms are able to produce more output at a lower cost. This means that firms are willing to supply more output at any given price, and the supply curve shifts to the right.
When MC increases, firms are faced with a higher cost of production. This means that they are only willing to supply more output at a higher price, and the supply curve shifts to the left.
Option B is incorrect because if MC remains unchanged, the supply curve will not shift.
Option C is incorrect because if MC increases, the supply curve will shift to the left.
Option D is incorrect because one of the factors that can cause the supply curve to shift is a change in the MC of production.