Which of the following is used by networks to cover production costs?

Commercials are a primary source of revenue for networks to cover production costs. When networks produce television shows or other content, they rely on advertising to bring in money. These commercials are sold to advertisers who want to promote their products or services to the audience viewing the network's content. The amount of revenue the network can earn from commercials is often influenced by factors such as audience size and demographics, making the show more appealing to advertisers.

The other options play different roles in the television industry. Focus groups are used for testing and gathering audience reactions, which can help refine content but do not directly generate revenue. Demographics refer to the statistical characteristics of a population, useful for targeting advertisements but not a revenue source themselves. Ratings indicate the popularity of a program and can affect how much networks charge for commercials but are not direct sources of funding like the commercials themselves.