Firms producing and selling a large variety of goods will generally follow:

Cost plus pricing
Marginal pricing
Skimming pricing
Product line pricing

The correct answer is: D. Product line pricing

Product line pricing is a pricing strategy where a company sets prices for a range of products in a way that reflects the different levels of demand for each product. This strategy is often used by firms producing and selling a large variety of goods, as it allows them to maximize their profits by charging different prices for different products.

Cost plus pricing is a pricing strategy where a company sets prices by adding a markup to the cost of production. This strategy is often used by firms that have a high level of fixed costs, as it allows them to cover their costs and make a profit even if they sell a small number of units.

Marginal pricing is a pricing strategy where a company sets prices based on the marginal cost of production. This strategy is often used by firms that have a high level of variable costs, as it allows them to minimize their costs and make a profit even if they sell a large number of units.

Skimming pricing is a pricing strategy where a company sets a high price for a new product in order to maximize profits from early adopters. This strategy is often used by firms that have a unique product or service that is in high demand.

In conclusion, product line pricing is the most appropriate pricing strategy for firms producing and selling a large variety of goods. This is because it allows them to maximize their profits by charging different prices for different products, which reflects the different levels of demand for each product.