The correct answer is: C. horizontal
A perfectly competitive firm is a price taker, which means that it has no control over the price of its product. The demand curve for a perfectly competitive firm is therefore horizontal, or perfectly elastic. This means that the firm can sell as much or as little of its product as it wants at the market price.
A downward-sloping demand curve would indicate that the firm has some control over the price of its product. This is not the case for a perfectly competitive firm.
An upward-sloping demand curve would indicate that the firm faces a shortage of demand. This is also not the case for a perfectly competitive firm.
A vertical demand curve would indicate that the firm faces an infinite demand for its product. This is also not the case for a perfectly competitive firm.
In conclusion, the demand curve facing a perfectly competitive firm is horizontal.